I just got a 680 which replaces sli'd gtx 275s. I can play arkham city @1080p with everything maxxed out but when using physx on high i definately see a performance hit. I thinking of using one of the gtx 275s as a dedicated physx card but have some questions.
- i've read here that you need to be careful the using two cards won't degraded the pcie speed from x16 to x8 on some motherboards. My motherboard is an older EVGA 132-YW-E178-A1. The marketing around the board states: "There are 3 PCIe x16 slots supporting 3-way SLI video. There is no downgrade of slot speed with this board, the 780i supports full x16 bandwidth across all three slots to take full advantage of your video cards". Does that mean i don't need to worry?
- i realize there are only a handful (if that) of games that take advantage of physx, so is it possible to shut the gtx275 off when not needed to save power and heat?
- would my performance be the equivalent of using the 680 alone without using physx at all?
First you need to know that when the board says that the slots will go from x16 to x8 when certian slots are occupied just means that there are a certian amount of lanes available and they split them up between the slots. You will not notice any difference from a x16 and a x8 performance wise and I have asked Evga support this very same question. The board that you have says x16 for all three slots because the 780i chip in the board supports the extra lanes. The ram is only ddr2 which would be more of a limiting factor than any x8 lanes. Also the cpu socket is LGA775 which is also kind of old and could also be a limiting factor as well.
I would use the GTX 275 as a physx card and that way you can see if by using it will make a difference. In the Nvidia control panel you have the option to enable or diable the physx option and on which card you want to enable it when you want to run the physx opotion.
The speed of the PCIe bus won't change, you'll just have fewer lanes to work with going from 16x to 8x. But this doesn't really matter that much I've seen a 2-4% to no decrease in performance going from 16 to 8 lanes. Check out reviews with a i7 2600k and GTX 680s in SLI and you'll see performance with two cards in 8x/8x with little to no performance penalty whatsoever.
The motherboard you have has an Nforce controller, so you don't need to worry about 8x anything.
You won't be able to turn off the 275 entirely when you're not using it. Every card I know of consumes power when idle.
When not using PhysX, your performance will be that of the 680 alone.