Things that always improve single PC multi-boxing.

1) More ram 8gb is good, 4gb is cutting it close for 5 toons.
2) Processor a decent Quad or i5/i7 and you'r set.
3) 1GB Videocard is fine, 2 1GB cards is best (but really prob overkill).
4) Harddrive speed I'd say a 7200RPM min, 10000RPM better, SSD Best (Pricey).
5) Turn the settings down on your slaves