at the point where your computer is sloooow new computers are often 100% faster (or more)
case in point a c2d geekbenching at 2500 and an i7 clocking 10k
at that point it doesn't matter if you got the 2.2 or the 2.4, they're both slow.
RAM is easy to upgrade so buy what suits you now, upgrade a few things when you feel left behind (ram/ssd) and then start saving for some new technology