Hardware upgrades won't help you very much. If your existing
drive is a 5400 RPM, you might get a little performance bump
going to a 7200 RPM. However, you're really limited by the
age of the Motherboard/Chipset. Even a memory upgrade from
256 to 512 isn't likely to improve performance. With prices on
new systems falling into the $350 range, you might want to save
your money and just go for a complete new PC.
For performance improvements, you should turn off XP's GUI
enhancements (Use Classic Theme), trim back System Restore
to a small allocation and make sure all your drivers are up to
date. On an older machine like yours, defragmenting the drive
on a regular basis will help. Sometimes a Motherboard, CPU
& Memory upgrade is advisable. With that you could get a MB
with on-board Video/Sound/Nic & USB2 for $70. You could
go for a AMD Sempron 2.4 Gigahertz for $75, 512 Memory
for $50. But you'd be at $200, so you'd be approaching the full
new system cost from Dell. Those prices are from my local
distributor and you might have to get a new Power Supply rated
300+ Watts. Without knowing your intended use, it's really hard
to advise you on the best approach.