Hard to say with Crysis 2 as it's not out.
crysis was a general system pig, high ram demand, high graphics demand and high cpu demand (mainly because of the physics system the engine uses which runs off the cpu, you can see this when you do something like demolish a shack, all the physics will cause a big cpu spike)
For your resolution I'd say that's a pretty good system
an upgrade of OS and an extra 2x1gb of memory (if you can get it for a reasonable price) would be a good move in the short term.
I'd also agree with CAT on a 2nd hand cpu, not nesserally a quad core, even an e6600 can be got fairly cheap and "should" hit 3ghz on your system without need for voltage increase (well I didn't on my e6600+MSI P35 Neo2-FR so I know it's possible
)
My big word of warning about fleabay is that most of the buy-it-now offers are just plain ripoffs, often far higher than you can buy from other e-tailers.
Acutions on 2nd hand stuff is where the bargins come from and this is where feedback can be far less useful, as these are sometime just people selling off stuff after an upgrade so they are not powersellers or sell much at all, so don't generate the feedback.
However these can often be the best people to buy from because they are just getting rid of stuff and trying to get some money back after an upgrade and not reselling to make a profit.
Above all, figure out a max price and don't bid over it, don't get into bidding wars, if it overs over your max just wait for the next one and always check current online prices first.