if you are after a little more speed but dont want to spend the money, the first thing you should try is to clean up your hard drive. Regardless of how fast the Cpu is, a full hard drive will make your computer dog slow.
partition your hard drive as 4 small drives not 1 large drive, and leave your old drive in and move the swap file to that drive.
256 megs should be enough memory you might see a small boost with another 256.
16 megs on your video card is a bit low for graphic intensive programs.
if you want to build a new system for the fun of it, or so you can do it slowly over time start at Toms hardware
http://www.tomshardware.com/
It's not the money so much as the hassle and fearing that I'd be getting in over my head.
Where can I find out how to partition the hard drive?