|Extreme budget build for your PC2-6400 RAM:|
AMD Sempron 140 Sargas 2.7GHz 1MB L2 Cache Socket AM3 45W Single-Core Processor - $36
ECS A780GM-A Ultra AM2+/AM3 AMD 780G HDMI ATX AMD Motherboard - $54 after rebate
ASUS EAH4350 SILENT/DI/512MD2(LP) Radeon HD 4350 512MB GDDR2 - $20 after rebate
With any luck, that BIOS will allow you to unlock the 2nd core on the Sempron & turn it into a dual core CPU. The onboard HD 3200 graphics aren't bad (comparable to the HD 2400) so adding a video card isn't an absolute necessity unless you feel you need more graphics power. The HD 4350 can be added to enable Hybrid Crossfire, or you can disable the onboard video altogether & add an even better card.
For $110 (or $90 without video card), you'd have a budget system that will kick the crap out of your current system. In fact, it would be faster than the fastest P4 system.