Doesn't make any sence. Should run just fine with both/three sticks in in any slot you put them into. The only mem that demanded the slot compliance issues was/is RAMBUS. Either it was not sugged down into the mem slot or, for some reason, the 512 stick has some issues. By the way, more mem is always a good thing! However, if your all hung up on running dual channel mem. just install the new mem cross sloted in the same position. It depends on the mobo.....actually. My intel D865PERL will run dc cross slot or same slot double.
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.