Newegg and Zipzoomfly are good. I just got myself a 6800GT and can vouch for that, though Im only expecting it to last me about a year or so. It should play next years games with no problem however. 6600 would be slightly limited over the next year i would think.
6800GTs bout $285 atm. The next step up jumps to over $400 with the 6800Ultra and X850XT-PE, which maybe not as good value for money seeing as they're already dated!
It depends what games are you're wanting to play neodudet?? Also, some dont like to run anything less than full res and highest settings, whilst others dont care as long as it runs!
