Do you play a PhsyX game on a weekly basis? If so, go with PhsyX.. otherwise go with Tri-SLI.
I honestly don't see the point of Tri-SLI though, it doesn't scale all that well, you'd probably be better just buying new cards and selling your old ones. Won't be long before the 275s can't max games, and they don't have DX11.
If you aren't NVIDIA Dedicated:
You could always grab a 5970 and use a GTX 275 for phsyX, that'd set you back $600 but you could sell one of your 275s.
I honestly don't see the point of Tri-SLI though, it doesn't scale all that well, you'd probably be better just buying new cards and selling your old ones. Won't be long before the 275s can't max games, and they don't have DX11.
If you aren't NVIDIA Dedicated:
You could always grab a 5970 and use a GTX 275 for phsyX, that'd set you back $600 but you could sell one of your 275s.