Can you Triple SLi a GTX295 with a GTX275? A friend asked me this I told him no, but after some thought I'm not so sure.
No sorry. All GPU's must be the same. The brand can be different but they all must be the same model (GTX 295 in this case). If you have those pre-overclock models and the others standard, that will work fine. Same applies for ATI Crossfire.
There's a bit of Leeway for the ATI cards, you can crossfire a HD4870+HD4890, but the effect is much more minimal and that the HD4890 gets downed to a HD4870.