Some prices form one of the online shops:
-GF8800GTS 320MB GDDR3: 320EUR
-HD2900XT 512MB GDDR3: 390EUR
-GF8800GTS 640MB GDDR3: 420EUR
-HD2900XT 1024MB GDDR4: 500EUR
-GF8800GTX 768MB GDDR3: 530EUR
Guess the 8800GTX is to be compared with the 2900XT 1024MB. I'll look for some decent up to date gf. vs. radeon cat. 7.8 benchs when there are some to get an updated overview of the situation which has IMO changed from 5/2007.