An almost 4 year old (not overclocked) system, but still able to play Crysis med/low settings with playable frame rates. Really want to get my hands on Far Cry 2 but it would seem that my X850XT won't cut it. Is this the case?

Rather than spend £1,000+ on a new rig (circumstances prevent me from risking a pc death due to a messed up major upgrade) thought about getting a hd 4850. Not too power hungry, but not sure whether my current psu has enough reliable power. Or whether the whole system has really reached the end of viable minor tinkering and I now need to spend some serious money.

Any advice much appreciated - Thanks