Adding ram to overclocked Abit NF7s/Buffalo Winbonds. Compatibility issues?

I've got an Abit NF7-s2 that has two 512mg pc3200 Buffalo Winbonds in it, stable at 2400mhz. I need to add another either another gig or 512 megs, but I'm not sure about compatibility problems I might encounter. First, is it asking for trouble to mix a 1 gig stick with 512s from another manufacturer? If I can go with a gig, or only with 512, is there a moderately-priced option that will keep my rig stable? Thanks for your advice.