If you are just into apps and IE using windows xp. 1gb of ram is plenty. Your pc only supports ddr ram.
DDR2 is totally different. It uses an different pin configuration and is not backwards compatible
Since your system is socket 754. there aren't any dual-core cpu's availble. So that is out of the equation.
So either save up for a s939 motherboard and cpu to still use your ddr ram. Or wait and build a c2d pc.