Re: Two Questions All!
Question 1 -I'm reading a lot of great reviews for the 7900gt. Overall, the 7900gt is better in all aspects, more pipelines, higher core clock, and higher memory clock. Both cards are gddr3 but I just think the 7900gt's are worth it for the price.
Question 2 - Don't get the Fx5500, thats just a crappy line of graphics cards. I'd go with the 6600. This Asus plays Bf2, HL2, Doom 3 plays pretty well. These are pretty much the most graphic intensive games I think of so when this card can handle them you know you got a keeper. Go for the 6600gt, if you got more money!
Rig -- Emachines T5026, 3.06ghz P4, 300W Bestec PSU, 160 sata hd, 512mb dual channel 3200 ram @ 3-3-5-8, ATI Radeon 9250 pci, dual layer 16x dvd+/-rw.