P4? Wow. That's some old school **** right there
.What Bishop said. That CPU is holding you back. You mind as well build yourself a new PC, because if you want to upgrade the CPU to something more recent, you're going to have to upgrade the motherboard, and then you would have to upgrade the memory, and at that point, you mind as well build yourself a new PC.
The price of building you're own has dropped in the last few years if that's the route you wanted to take. Mine is around 2 years old, and was updated to the most recent hardware/cpu/mobo/memory at that time, and it ran me ~$500-600.