Your machine is good, but if you wanna run 5 on one comp I STRONGLY recommend moving up to a quad-core instead of dual-core. Distribute the load a little better. Card should be fine, ram might be fine but if you can slap another gb or two in that would certainly help; also you may want to consider moving your copies to multiple hard drives if you have them, 1 wow seeking per drive is faster than 5 wows all fighting each other seeking the same thing; your HD only seeks so fast :p

4gb is enough to run anything nowadays, don't go past that if you don't have to. Though 4gb will indeed require Vista and/or a 64bit OS due to xp/32bit simply not recognizing/properly utilizing more than 3gb or so. Though yours IS DDR3 and I have no idea how big the performance difference is vs DDR2...DDR3's too new for me to know alot about yet lol