I would skip odd numbers with nvidia. I mean the Geforce 2 was so good that they still sell them even today. Where can you find geforce 3's , ebay and probably no where else. Geforce 4 MX has not gone away either. FX series well the high end cards are legacy but the buget cards still have life in them, though since it's odd number it did not fair too well against ATI. Geforce 6 series is a great triumph again for nvidia. The 6 series will live on and probably totaly replace the Geforce 4 series in the future. Geforce 7 is odd number, so I will just be somthing that only a niche group of uber geeks will buy. I'll wait for 8 series
ATI is a bit different. They don't ever flop, but they totally dominate on Odd numbered series. Yet they still sell a lot on even numbered ones as well, but since nvidia's even numbered cards are so good, they have more feirce competition.