That's almost a 10 year old system, your money is better spent replacing it rather than upgrading it. A newer system with a much faster dual core, Q model quad core, or maybe even an Core i3 CPU can be found for about $100-150 which is about what a CPU and video card for your system would be.
If you really just want to upgrade it, you will be limited to very slow low power video cards like 8400 or 210. CPU, it should run the E6700, just make sure you do a BIOS update, that CPU is in the same model range as yours, or possibly an E8600 which is the same socket and power use but is a different generation.