Now the question is which of these options is the best one? Buy an extra hard disk or a new computer?
You said you need more ram. There is a huge difference between ram memory and a hard disk, a new hard disk will not solve this problem, you gotta buy more ram....
http://en.wikipedia.org/wiki/RAM
To buy some more ram isn't expensive and it helps to speed up your PC over all (not only for pc games) .... however, we can't be sure that this will solve your SIM problem (maybe it doesn't run because you don't have 2.4 ghz...?)
So well, it's up 2 you if you want to try 2 update your ram memory or buy a new Pc, it's hard 2 tell, it depens a lot on how much use the PC, if you'd need a new Pc anyway.... like sofi stated, 2 buy a new pc only for Sim 2 seems a lil wreid, but maybe it's time for a new pc anyway and that's only a good reason to buy one.