GFx card wise the 768meg nvidia gtx460 is absolutely the best value for money card around. Performance wise it's more or less on par with the radeon 5850, but for a little under $200 (5850 is about 330ish). Further down the track youre probably better off with a nvidia gtx450, which can be picked up for about $130, and comfortably outperforms the radeon 5750, which is closer to its price point, and trades blows with the 5770 (ie. theyre both faster for different software), but the latter is more expensive. Until the last month or 2 ATI was definately the way to go, but with the introduction of the nvidia gf 104/106 gpus the balance has shifted. As for the cpu, it depends on the socket. Id guess its either a socket 754 or 939 cpu (otherwise it'd most likely be using ddr2). Unfortunately the faster speed socket 939 cpu's have held thier value a little, so wont be the best value around, but you should be able to pick up a 2.6ghz athlon x2 5200+ for about $50 Id imagine. Speed of RAM will also have more of an impact than going beyond 2gig, but obviously additional ram never hurts either. DDR1 is comparitively pricey now though, so buying 2 gig ddr400 isnt too much more expensive than a new socket 775 mobo+2 gig ddr3 1333.
All in all something like an e5400+cheap mobo+2 gig ddr2/3 (or athlon2 250/am2+/3 for amd) wont be much less than buying new ram and cpu for your current system so I guess it depends on exactly how closely youre watching your wallet. Personally I'd probably just sell the current ram+mobo+cpu and fork out the difference for cheap new mobo/cpu/ram. Cheap these days doesnt mean bad, generally just stripped down feature wise compared to the fully fledged older borthers (and even then not necessarily stuff everyone would notice). Overclocking will also usually reap better results on a more expensive board.
