They have to be in matched pairs (2x 512 etc). 2 will run in dc and 3 will run in sc. add another 512 like Chirs just suggested above and run 2 gig in dc. Memory is cheap for now so go for it!
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.