Your old system is either Intel based (Pentium) or AMD.

Upgrading an old pc is not a very good idea.
You will need:
- board
- CPU + Cooler
- Memory
- VGA card (old one does not fit)
- Power supply (there are converters 20pin to 24pin
but the output will be to low)
- Hard disk, boards now use SATA instead of PATA

The only thing you can keep is the case, witch I would replace because of the cooling requirements of new systems.

If you have little experience in building pc’s the best option is to buy a complete system.
The only reason for building one yourself is that you can customize it.

Rob