I would bet you would like two 1 gig sticks in there at least. Remember that if you use 4 512 doubled side sticks with the current AMD64's (not sure about the new core revision coming out imminently) you will go to 2t memory timing and lose about 5-10% performance versus 1t. I would get the 939 variant of Ati, Via, or NVIDIA because they should all be dualcore compatible. The 939 AMD64 dualcores appear to be arriving around June and you could go from a ( 3500+ (2200mghtrz 512k L2 cache) single core to a X2 4800+ (2 2400mghrtz 1024K L2 cache cores!) dualcore and watch your multithreaded performance skyrocket. Obviously, you are trading or purchasing Win XP 64, or a 64bit Linux OS right?
I would keep the Raptor for your root and program and swap drive if possible and grab a couple larger SATA drives for your data files. I wouldn't necessarily use raid myself but do you have an external or DVD-RW backup system. I learned the hardway to back up externally, LOL.