RAM, RAM, RAM and some more RAM.
The modern MB and 64bit OS can handle 8GB. Might be overkill, but planning to go to 8 GB is a better strategy than just buying 4GB with no long term strategy. I would buy 2 x 2GB sticks with the plan to go to 8 GB if needed or desired. The 2x2 sticks will cost a little more, but allow you to bump to 6 or 8GB at a latter day with the lowest total cost.
The Q6600 is an ideal CPU.