Run Ad-Aware and clean out any spyware, run some up to date Antivirus software (preferabley a good one, such as McAfee) and clean out any viruses, then defragment your hard drive.
If it doesn't speed up at least a little bit once you've done all that, then I suggest backing everything up and doing a fresh format/install.
If you're looking to upgrade your hardware, upgrade your graphics to at least 128MB DDR, and upgrade your CPU to something like an Athlon XP 2500+ [Barton Core] (check your motherboard will support this though, because it runs at an FSB of 333MHz). At least an 80GB 7200RPM hard drive wouldn't go a miss either. Your RAM is fantastic, so there's no need to worry about that.
Overclocking-wise... Read up on this subject A LOT before you even start to consider it. On the page file size front, I'd say set it at about 512MB - 1024MB, or even higher if you wish, although that should be okay.