The next step for you would be a complete system upgrade. I'm sure that case won't hold most of today's new motherboards, and if it does, you would need a new powersupply. Even if you could fit a modern mobo in your case, it won't use your old CPU, ram or anything else anyway.
If you like tweaking, head on over to Newegg and buy the following:
Whatever case you like with a minimum of a 350watt power supply
Budget System:
Motherboard: If you're looking to go budget go with an Athlon XP (Socket A) motherboard, something like a DFI Infinity (based on the Nforce2 chipset), the Abit NF7-S or something similar would be best.
CPU: Get an Athlon XP 2500
Ram: 512 megs minimum PC3200 ram.
Video card: Look for anything in the Radeon 9600 or 9800 series, or if you don't like ATI go for Nvidia, anything the GF FX 5600, 5700 or 5900 series.
Hard drive: 120 gig, preferably SATA, ide is fine too.
If you don't want to tweak, buy a Dell.