you can upgrade now for about 100 quid. thatll get you a 3 gig cpu and a mobo for it.
spend another 200 on a very decent graphics card. and youll have a pretty quick machine.
of course you can spend a lot more, but unless youve got a special application to run, i wouldnt bother.
ie a dual core 3 gig cpu will cost you about 600 quid. a big difference for a domestic user.
generally, HW these days is way in front of the SW.
there is no point in overclocking these days, the price of fast cpu,s is just so cheap.