First, any chipset will do if you use only one card (even dual-gpu cards like 4870X2 or 9800GX2). The 750i/780i/790i/X48/X58 cost more, but offer multi-card support (SLI for Nvidia and CrossFire (XFire) for ATI). For NVidia cards, you need an NVidia chipset (750i/780i/790i). For ATI cards, it's Intel chipset (X48/X58).
So, do you need SLI/Crossfire support?