Hardware-wise:
RAM is the most important
plus, it's the simplest upgrade you can perform that in some cases is just as good as a processor upgrade

Software-wise:
Disk defragmenter
Use programs such as TweakXP or registry hacks to speed up the NTFS file system

Registry hacks? details...

www.blkviper.com

Turns off services and such. More oriented for NT based systems, but has a few 9x tips available.

Though I agree with more memory never hurting anything, there is a practical limit (for most) of 256MB. Any more than that and either you're a geek, person with money, or don't know what's going on in the first place and had someone build it for you. 256MB is plenty for any normal user with today's software; not applications that came out 5 years ago.

I say that, then I have to bark about the fact that there's no substitute for raw processing power. In general, the more GHz you have, the faster the system. A faster bus makes up a healthy chunk of this. I would rather have an AMD system on a 200MHz bus at 2GHz than one with a 100MHz bus at 2GHz.

http://9337387.home.icq.com/index.html