Yes, there are so many wireless products out there. I'll help you sort them out:
Entry level: Usually $300 and below.
Not much in this price range that has decent sonic quality OR decent reception. If you are looking for a pro piece for a pro singer, steer clear of units in this price range.
Entry Pro level: Usually between $1000 and $400
In this range, there is a lot to choose from. To choose best, consider the parts separately;
Mic Capsules; Top performers are Sennheiser, Audix, AKG, Beyer, and Neuman
Transmitter/Receivers: The top dogs are Sennheiser and Shure in this regard. Audio Technica has a few, but in this price range they still don't measure up to the Sennheisers and Shures.
Pro Tour level: Usually $1000 and up
Just about anything in this price range is good. Some are better than others, but most will perform stupendously.
Your lady's voice does not need the vocal 'presence' enhancement at 4.5khz that the Shure SM58's and SM57's are noted for. Actually, the presence peak makes her voice harsh and brittle especially at high volume head voice. I'm not at all a Shure fan. I think their mics are Ok if you want something that you can drive a truck over and it still works, but I feel the sound quality is mediocre at best, especially at the SM58 level. Many, many women singers are paired with either Sennheiser or Audix cartridges because of this. Sarah McLachlan uses Neumann and Audix mics, and the KD Lang and Joni Mitchell tours are using Audix. Your lady's voice also has a wider dynamic range than most singers I've worked with. If you like the Shure line, the SM87 is what Christina Aguilera uses specifically because it can handle the dynamic range. But if you listen to her live, her voice lacks the detail and brilliance of Sarah or KD. I think her production crew sacrificed a little sonic quality to handle her freight-train pipes !!! ;-) Along those lines, the Sennheiser 865 capsule is similar to the SM87 in that it is a condensor mic and also able to handle the dynamic range very well with more detail. Sting uses Sennheiser mics on tour.
The Sennheiser EW135 G2 is what we use. This couples one of the best transmitter/receiver combinations with a Sennheiser 835 capsule.
Our singer also has a huge voice, and the Sennheiser 835 capsule works great even though it is not a condense mic like the SM87 or the 865. I got ours for about $500 each including tax and shipping. The same transmitter/receiver combination with the better 865 capsule will run another $100 or so. The couple great things about the Sennheiser EW1xx series is over 1400 channels to choose from, a metal 1/2 rack sized receiver, outstanding signal to noise ratio, and the ability to change cartridges easily if you want something different down the line. Shure's analog at the Entry Pro Level is the ULPX. A SM87 cartridge/transmitter/receiver in the ULPX series will run you about $900.
Now ... the bottom line. I suggest what we use; Sennheiser EW135 G2. Do a google search for comments on it and you will see glowing reviews. In all the years we have used ours, we have had next to no dropout. And in our live recordings, you cannot tell it is a wireless mic. You can usually find them on eBay (new, of course) at around $425. They are also available through zZound, SameDayMusic, Musician's Friend, and a bunch of other places. Going price lately is about $480. If you want the absolute best specifically for your lady's voice, go the extra and get her the EW165 G2. Her voice, while huge and powerful, needs some clarity help in the 8-10khz range especially when she goes into head voice. (I had to tweak it all the time on the mixing board) The EW165 G2 comes with the better 865 condensor cartridge that has the high-end clarity she needs and the same killer transmitter/receiver as the ones we use. The EW165 G2 is going for about $650 from most online stores, and about $500 or so on eBay.
Last recommendation ... If the prices a little steep, stay with a good wired mic for now. A SM87 goes for a little over $280, and the 865 (which I think is way better than the SM87) is about $230. The absolute bottom line in a mic should be getting great vocal tone. Don't compromise.
