On top of which ever card you buy, judging by your current spec; you will need to buy a new power supply as 350W won't be sufficient and it will also require RAM upgrade if you wish to place those game as 1gb will struggle to run smoothly.

Not sure on the graphics card choice but I would suggest that first stop is more RAM, it is cheap as chips and should give you a good boost on those games.

I run a 1.8Ghz Core 2 Duo, 3gb of ram and a HD3850 and get good performance for those games at 1680x1050 (satisfies me anyway), you have to turn down the graphics to get crysis (and by extension crysis warhead and far cry 2) running well but ... who doesnt?!

From all accounts the 3850 and 4670 are similar performance, so with that card I'd say you can run these games and games in years to come, as long as you have more ram, and from a quick perusal of the web, the 4670 also beats the 8600GT.

Would definitely side with the 4670 for midrange oomph. Generally better performance levels, driver support is better (slightly ironically!) and you've better IQ and features to boot.