If you can't afford a new box, wait a little bit. 128MB RDRAM would be good, but still just a patch to a problem--an old P4 will suck for general use. A Compaq will be bogged down on top of that, as well. And don't get another Compaq or HP . Everyone I know who had one has loved their non-HP or COmpaq PCs when they upgraded. I worked on quite a few of them, and...they were all bare minimum crap. Even the Dell 2400 series is nice in comparison.
So you're not in the US? Well, I guess see what you can do, but here's what I managed in the least few minutes:
-Biostart NF2-400 mobo. Most TNT2M64s were AGP 4x, IIRC, so it should still work fine. If not, Radeon 7000 cards can be had for under $40 USD.
-Athlon XP 2000+ retail (cheapest retail CPU).
-Geil 512MB PC2700 RAM
-Hitachi 120GB HD. This and RAM will make the difference for general use. I recently went from a 20GB 5400 RPM HD to a Spinpoint 120GB...very nice.
-Evercase black e4252 w/ 300w PSU. Nice price, the PSU will be fine for this kind of setup.
Ended up at $341 USD. That does not include Win XP. I don't know what SG is, but if you're a student, you might be able to get a full (non-OEM) copy of XP Pro for less than the OEM sells for--look into it. Some colleges and tech schools have it cheap. Otherwise, XP Home should be no more than $100 USD, but if you're in school for IT stuff, you'll likely want to hit yourself in the head at some point for not getting Pro.