ya you can run Crysis, and all other games with it. but Crysis and games close to it will need resolutions nearing a 17, or 19 inch screen and probably on medium to low settings.
Actually the 8400 is really close to the same as the 8600GT.. hardly any difference. what you'd do if any is instead of getting a 8600GT. get a 8800GT. as the 8800GT is at least double the performance of the 84, or the 8600GT's.
The 8800GT is just that much better. Also if your C.C is anything like the one by my house. They are WAY ovepriced. and that 8400GS was probably around $120, or more. and at Circuit city the 8600GT is around $150.
on Newegg.com the 8800GT is now down to $110.. You're best bet if it's worth the 3 days wait for you would be to return to CC if can. and online buy the 8800GT.
that might not be the way yours. is. but mine is way way overpriced. just a thought. but you should probably be ok for mild things.