DDR2 memory is expensive. I would suggest you either buy an 8GB kit (between $100 and $150) but you can try buying a 2x2GB kit to supplement your current one instead of replacing... However I can't recommend that. Mixing RAM kits is not guaranteed to work, especially if you don't get a kit that is identical to the current one.
DDR2 pricing is unlikely to ever go down so you might be better off holding off on upgrading and replacing the motherboard/CPU/RAM with new parts sometime next year, preferably after Ivy-Bridge's launch supposed to happen in mid 2012.