|"are you sure?"|
If you want a decent gaming experience, you should try to meet the recommended requirements, not the minimum. The E5200 is a dual core CPU that runs at 2.5GHz...the recommended minimum is a dual core at 2.0GHz, so you have that more than covered.
The minimum RAM requirement is 1GB, the recommended is 2GB, so you're dragging your feet in that department.
The video card is the key ingredient. You could have a 3.2GHz dual core & 4GB RAM, but if your video card is a piece of sh!t, gaming is gonna suck.
The 8400GS is a low end / entry level card...more of a general purpose card than a gaming card. You *may* be able to play some games with it, but Crysis isn't one of them.
Don't waste your money on a new CPU, it isn't gonna help. Invest in a decent gaming card 1st...you can add more RAM afterward. One word of warning though - gaming cards require a fair amount of power, so that means your power supply has to be capable of handling the extra load. If it isn't, you'll have to upgrade it & that will cost $50 or more for something decent. If that's not in your budget, just buy a Radeon HD 4670.
Have a look at this Crysis chart to see where the 8400GS ranks. I'll give you a hint...scroll WAY down:
Now look where the Radeon HD 4670 ranks & notice that even the top cards in this category only get 35 frames per second. You can get an HD 4670 at newegg.com for less than $60 after rebate.