Hi Mark: This month's Consumer Reports has a write up on cordless drills, as does the (I believe) July issue of Fine Home Building.
How much power you need depends on the work you plan on doing. 9.6 volts for occasional work around the house should be fine, but a big construction project will require a 12 Volt at a minimum, with 14.4 Volt preferable. IMO, 18 Volt is for the big stuff.
Personally, I own and have used the following cordless drills: Makita 9.6, DeWalt 12.0, Milwaulkee 12.0 and old B&D 7.0. Of those my hands down favorite is the DeWalt. It is extremely powerful, well balanced, easy to manuever and its battery seems to give a lot of life. The Makita was my first serious cordless and has served me faithfully for many years. But like Steve said, sometimes 9.6 just ain't enough. I only shorly owned the Milwaulkee 12. Now, I have been a faithful user of quality Milwaulkee tools for many many years, and perhaps this explains my high expectations for the cordless drill. But I was very underwhelmed by this tool when I first got it and used it about 1 1/2 years ago....so much so that I took it back and exchanged it for the DeWalt. Poorly balanced, poor chuck speed control and a keyless chuck that slipped.......Hopefully, Milwaulkee has improved this important tool. But Steve's point about getting 2 batteries is right on the money...and a rapid charger is also very nice to have.
Others that have recieved good ratings that I have not personally tried include the Porter Cable and the Fein...although I understand the latter has now been discontinued. Also, unless you will be doing very light duty and infrequent work, suggest you stay away from the B&D and Craftsman cordless products, or anything else advertised for $49.99.