How should I install a 1gb ram card ?

I know I should have matched cards but I didn't I bought a system, sight unseen that has 2 - 512mb cards in it, and I bought a single 1gig card, what I need to know is should I put the two 512mb cards in xmm1 and xmm2 slots and the 1 gig card in the xmm3 slot for it to read the 2 gig total, the computer maxes out at 4 gig, incidentally it has a 3.2GHz CPU... It's a HP Compact dc7600 series ATX tower...