Core 2 Quad Q9650 is the best CPU with an BIOS update.
The RAM your motherboard supports is DDR2 PC2-5300, DDR2 PC2-6400, DDR2 PC2-8500, DDR2 (non-ECC) and the maximum it supports is apparently 4GB (2x2GB).
You should also consider upgrading your GPU if you want to play the game maxed out at decent resolutions.