This is looking at the desktop/mini-tower configuration.
Socket 478, 400 MHZ & 533MHz
DDR 266 (PC2100) RAM, max of 512MB per stick, max of 1 gig
For RAM, go with two sticks of this:
$77.56, including shipping.
For CPU, go with the Pentium 4 2.8A Prescott 533MHz FSB 1MB L2 Cache Socket 478 Processor:
$188.99, including shipping.
This will basically max out your system's possiblities.
You may want to look for an AGP x4 video card. That would help your computer's performance in games.