http://www.xbitlabs.com/news/video/display/20050627185233.html
Still, buying one GeForce 6600 and adding another one after some time may be a feasible upgrade path as NVIDIA promised that from now on its multi-GPU technology will allow graphics cards with different BIOS versions and from different manufacturers to work in collaboration, thanks to new drivers. Up to now graphics cards from one maker and with similar BIOS versions were needed for multi-GPU operation.