I posted a long explanation of wireless systems but can't find it with the search utility. So I guess I'll write it out again and host it this time on my webspace.

The 2 things you need in a wireless system, in order of priority, are:

1) Reliability
2) Sound Quality

#2 is something you should be able to test out with any system at most music stores. So I'll focus on #1.

The FCC has licensed wireless audio systems to be used in the same bands as VHF and UHF tv stations. Your wireless system is legally required to accept any interference from these sources. As such, it is ideal to use systems that are set to frequencies that are either unoccupied by TV stations, or occupied by TV stations that are far away enough that at your receiver's location your transmitter's signal will be stronger than the TV station.

However there is another source of interference with wireless audio, and that's other wireless systems. So within the scope of #1 is #1.5, Reliability when multiple systems are used. To use multiple systems together, each has to be operating on a frequency clear of TV stations, other wireless systems, and the harmonics that are created by transmitters working in close proximity to one another.

The last one there gets a little complicated, to understand it I'll draw an analogy. When you tune a guitar you usually pluck the nearest string on the same note as the string you're tuning. When they're not in tune you hear a "beat". This beat is actually another note created by the two strings interacting with each other at different frequencies. Wireless transmitters do this as well, and the "note" they create is at a different frequency than the two transmitters, and will interfere with any transmitter/receiver on that frequency.

So lets cut to the chase. To avoid all these problems you need a wireless system that A) works within a band free of TV stations in your area (and areas you might gig in), B) able to change frequencies within that band, and C) designed to work with the number of systems you want to use.

Other cool things to look for systems that can scan for useable channels and automatically sync the transmitter to the receiver with infared (you push a button, point the IR ports at each other, and you're done). Also 9V batteries have become much less popular because AA systems last longer and the batteries are cheaper.

Before I make any specific model reccomendations let me preface myself by admitting that I indirectly work for Shure, and that I am biased towards their products and models since I support them and them only every day at my job. Since that's my area of specialty I will tell you what Shure has to offer, you can feel free to explore other brands all you want but I'm not an expert on those so I'll leave it to someone else to compare those.

Shure has 6 main lines of wireless systems. Here are the basic differences between them.

Line: PG Wireless
Max systems (in ideal conditions*): 8
Frequency agile: YES
Pros: It's inexpensive, sounds good, and works with a few systems.
Cons: As of 07-24-06 they are still hard to find as they're new. The transmitters and receivers have to be manually set to their channels and do not scan for the best frequencies. Also the handheld mics only ship with PG58 heads instead of SM58. They also use 9v batteries.
Minimum Advertised Price: $300 for a single system, $550 for a dual.

Line: PGX Wireless
Max systems (in ideal conditions*): 12
Frequency agile: YES
Pros: You can use more together than PG, the transmitters and receivers scan/sync their frequencies with IR, and you can get the handhelds with PG58, SM58 or BETA58 heads. They also use AA batteries (cheaper and last longer than 9v).
Cons: You have to use a tray to put them in a rack, if you were to go on tour with multiple systems you could run into interference in different areas.
Minimum Advertised Price: $400 for a single handheld system.

Line: SLX Wireless
Max systems (in ideal conditions*): 20
Frequency agile: YES
Pros: Ready to be toured with, are rack mountable, AA batteries and IR scan/sync. Handhelds come with SM58, BETA58, or BETA87 heads.
Cons: As with PG and PGX, the receiver and transmitter are made of plastic, when touring you probably would preffer metal. Not as affordable for bands on a budget.
Minimum Advertised Price: $599 for a single handheld system.

Line: ULX(S/P) Wireless
Max systems (in ideal conditions*): 40
Frequency agile: YES
Pros: Super reliable for multiple systems, systems scan/sync up with IR. The "P" (professional) versions come w/ metal receivers as opposed to plastic, and both "S" and "P" models are easily rack mounted. Available with SM58, BETA58, and BETA87 heads.
Cons: This is an older design, and still uses 9V batteries. Transmitters are plastic. Expensive.
Minimum Advertised Price: $659 for a standard handheld system, $829 for the professional.

Line: UHF-R Wireless
Max systems (in ideal conditions*): 108
Frequency agile: YOU BETCHYA
Pros: Top of the line wireless systems. You'll see these on the riders of most major bands and music groups, as well as in churches, universities, boardrooms - you name it. Systems are ethernet ready and can be programmed remotely via software, they also scan all bands and the software allows you to graph wireless band activity over time to plan ahead for events. Receivers and Transmitters are metal and will take severe abuse, and the systems use AA batteries. Available with all the popular Shure wireless microphones, including the new KSM9 "studio quality" condenser (it's actually a damned nice mic, all hype aside).
Cons: Expensive.
Pricing: Thousands of dollars per channel.

* What are "Ideal conditions"? When have you ever experienced "ideal conditions" at a gig? When it comes to wireless, if they can't hear you because of interference then you wasted your money. If your gig is more than 2 miles away from where you rehearse then you better be prepared for unfamilliar RF interference. Under those circumstances I would never want to be using the maximum number of systems in "ideal conditions". Always leave yourself as much wiggle room here as you can afford, if you don't you will regret it.

So after that deluge of information, here's my advice. For a local band I would reccomend PGX systems, for an up and coming band that's going on the road I would reccomend SLX systems. Both lines scan for the best frequencies, use AA batteries, and sync up with infrared. This means that if something goes wrong you are a button push away from fixing the problem by syncing to a new frequency, and your battery budget wont hurt as bad. PGX and SLX also use the same "Audio Companding" technology as the UHF-R systems, which means the sound quality is on the same level as Korn and Dishwala and Duran Duran and all those other bands with funny hair and pants that are too tight.