975X is the newest chipset out of Intel. the one you mentioned is 955 (its pretty old), 975X is far most only to accept 2 graphic cards (ATi Cross Fire Editions only).
or, even you can go for 965 series chipset, it as well as good as 975. as recently Quad Core Intel is rested on 965 series chipset.
also, if you are looking for nVidia's Dual, then nForce4 is the ultimate choice, as NVidia's chipsets wont support ATi cards, and Intel's own wont support nVidia's in Dual Assembly..
also, 1 PCI-E car will work @ X16, whilst 2 will work on X8 each. do your maths. using 2 cards wont bring significant performance upgrade.
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"