Okey, first on the X600 VS 6600GT:
1) the X600 is basicly a 9600XT in a PCI-E core, so unless your upgrading your mobo to a PCI-E one, its worthless.
Also, the 6600GT is ALSO PCI-E ONLY, neither are an option for a reguler upgrade because neither will have an AGP option.
2) the X600 is outdated in favour of the X700, which losses badly against the 6600GT no matter what test you run(see Ananadtech.com, Tomshardware.com,Xbitlabs.com)
3) For an upgrade, there is nothing more efficient then a 9800 unless your going for a 6800GT.
4) ATi won the last generation(FX and 9xxx), it has allready lost badly against the 6xxx line of cards that nVidia put out, to say nothing of future preformance.
5) The 9600XT is overpriced, and underpowered, for its price, you can get a 9800Pro today.
6) Unless your getting one of the new cards (6xxx or Xxxx) there is no need for 256MB of memory on your card, becuase you don't have the power to run in resolutions that use that texture size.