just another GX2 for quad unfortunately.
"In an SLI configuration, cards can be of mixed manufacturers, card model names, BIOS revisions or clock speeds. However, they must be of the same GPU series (e.g. 8600, 8800) and GPU model name (e.g. GT, GTS, GTX) [16]. There are rare exceptions for "mixed SLI" configurations on some cards that only have a matching core codename (e.g. G70, G73, G80, etc), but this is otherwise not possible."
possible maybe thru some awesome driver mods? Solaris?
EDIT: wait maybe a 9800GTX with the 9800GX2?
NVIDIA SLI-Ready GPUs:
NVIDIA GeForce GTX 280*
NVIDIA GeForce GTX 260*
NVIDIA GeForce 9800 GX2**
NVIDIA GeForce 9800 GTX**
NVIDIA GeForce 9800 GT
NVIDIA GeForce 9600 GT
NVIDIA GeForce 9600 GSO 512
NVIDIA GeForce 9600 GSO
NVIDIA GeForce 9500 GT
NVIDIA GeForce 8800 Ultra*
NVIDIA GeForce 8800 GTX*
NVIDIA GeForce 8800 GTS 640
NVIDIA GeForce 8800 GTS 320
NVIDIA GeForce 8800 GTS 512
NVIDIA GeForce 8800 GT
NVIDIA GeForce 8600 GTS
NVIDIA GeForce 8600 GT
NVIDIA GeForce 8500 GT
NVIDIA GeForce 7950 GX2**
NVIDIA GeForce 7950 GT
NVIDIA GeForce 7900 GTX
NVIDIA GeForce 7900 GT
NVIDIA GeForce 7900 GS
NVIDIA GeForce 7800 GTX 512
NVIDIA GeForce 7800 GTX
NVIDIA GeForce 7800 GT
NVIDIA GeForce 7600 GT
NVIDIA GeForce 7600 GS
NVIDIA GeForce 7300 GT***
NVIDIA GeForce 7300 GS***
NVIDIA GeForce 7300 LE***
NVIDIA GeForce 7100 GS***
NVIDIA GeForce 6800 Ultra
NVIDIA GeForce 6800 GS
NVIDIA GeForce 6800 GT
NVIDIA GeForce 6800
NVIDIA GeForce 6800 XT
NVIDIA GeForce 6800 LE
NVIDIA GeForce 6600 GT
NVIDIA GeForce 6600*
NVIDIA GeForce 6600 LE***
NVIDIA SLI-Ready Quadro GPUs
NVIDIA SLI-Ready Motherboards:
NVIDIA nForce 790i Ultra SLI
NVIDIA nForce 790i SLI
NVIDIA nForce 780i SLI for INTEL
NVIDIA nForce 780a SLI for AMD
NVIDIA nForce 750i SLI for INTEL
NVIDIA nForce 680i SLI for INTEL
NVIDIA nForce 680i LT SLI for INTEL
NVIDIA nForce 650i SLI for INTEL
NVIDIA nForce 590 SLI for AMD
NVIDIA nForce 570 SLI for AMD
NVIDIA nForce 590 SLI for INTEL
NVIDIA nForce 570 SLI for INTEL
NVIDIA nForce 500 SLI for AMD
NVIDIA nForce Professional
NVIDIA nForce4 SLI X16
NVIDIA nForce4 SLI
NVIDIA nForce4 SLI XE
Intel Desktop Board D5400XS
*NVIDIA GPUs that support 3-way SLI
**NVIDIA GPUs that support Quad SLI
***NVIDIA GPUs that do not use the SLI connector.