In response to your original query, yes, please get the 7600GT. I believe it is worth the money and superior to the 6600GT. Any case will do, but try getting a 400watt PSU for this video card or more.
FOr the video card, I really like this one. It's the OC version and it's got a huge rebate on that sucker. So get that if you can right now:
eVGA 256-P2-N554-AX Geforce 7600GT 256MB 128-bit GDDR3 PCI Express x16 Video Card - Retail