The P4 is half as fast clock-for-clock, core-for-core than Core2 that came after it so it really is not worth sidegrading to. If you want a major boost on a budget while remaining with Intel, I would recommed going for a Pentium G2120 and a b75/h77-based motherboard. This will use half as much power as a Pentium-D and be 5X as fast as your current PC. Total cost assuming you reuse everything else would be around $250 for parts.
However, since your PC is almost certainly using a BIOS-locked OEM copy of Windows, you would need to throw in an OS as an extra.