X200 is the most limiting factor there, and your power supply most propably. Adding RAM will be very much less expensive with a new mobo(examples:
http://www.newegg.com/Product/Produc...82E16820145184 http://www.newegg.com/Product/Produc...82E16820148212 ) Also you can get a decent dual-core AMD for 57$, and maybe upgrade later, but bear in mind that AMD currently doesn't offer any gaming powerhouse CPUs. Low-end gaming goes fine with a athlon 64 x2 5000+-6000+, but for high-end gaming, Intel E8500 Q6600 and Q9550 take the win on every aspect.
However upgarding your mobo will force you to reinstall Windows, and if you have a Dell/HP/PP, etc, you'll need to buy a new copy of Windows.
I woud upgrade you hard drive too, ATA drives
will slow the games down compared to a new SATA2 7200 rpm 32 MB cache drive.
Other parts: Radeon 4670 is good for mid-high gaming, and can be run with a 40 $
QUALITY PSu.