If you like the Nvidia cards, go for a 6600GT. You will bypass the issues the 5xxx series have (although I have learned to live quite happily with mine). The 6800 is a better performer, and if you can afford it a better choice, but the 6600 is a good step if you are on a budget that prevents the 6800. If you go ATI, 9800 or x800.
And definitely get the extra RAM.