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GTX 275 is a good GPU but only supports DirectX 9 & 10, you won't get any benefit from the DX11 current and upcoming games.
2 GTX 275s in SLi will consume a lot of power and run hotter.
Besides, it's the old GDDR3. GDDR5 is much faster and better. 896 VRAM would be an issue for VRAM hungry games and higher resolutions.
You won't be able to play BF3 with the 275 and that may be the case for most games coming out now and in the future. This article was posted yesterday here at Toms; http://www.tomshardware.com/reviews/battlefield-3-graph...
So if you want to play older games that have already been released in the past few years you will benifit from another 275.
Another GTX 275 wouldn't cost $160. You can find them on eBay for $75 dollars; I am selling mine for $75, but they won't do you much good in BF3. If you really want to play that game and are on a strict budget then definitely invest in a GTX 560 Ti or something of equal performance.
You'll be able to play BF3 and many future games with GTX 275 SLI, but you might have to wait for something to be done about those shadow-tearing issues.
Your proposed SLI setup would probably equate to roughly 560 Ti / 6950 performance, potential Vram bottlenecks notwithstanding. You might find it easier in the long run to go for the 560 Ti and sell your 275 (or keep it for PhysX!).
I'd hazard a guess that neither would run BF3 well on ultra.
You can keep the 275 for a dedicated physix card provided you have the slots to do so. If you go with another card like the 560ti and you want a second for sli then you need three slots for video cards. But if you only go with one new card then the 275 will function as a physix card.
I actually have three slots on my motherboard. I was not aware of the possibility to use a dedicated card for PhysX. I only knew SLI needed two cards with same specs to works properly.
Is there any real benefit to use a PhysX dedicated card? Does the system automatically uses the slowest card for PhysX or it is something I have to set? Also, I assume this feature would only work with games that are PhysX enabled like Batman and the like...
Your proposed SLI setup would probably equate to roughly 560 Ti / 6950 performance.
That's not true, the GTX 200 series are compared with ATI HD 4000 series, the implementation of the DX11, tesselation and GDDR5 in the HD 5/6K series are much better and stronger than the old GPUs technology.
I don't see any drawbacks prevent you from getting high FPS with SLI setup, even on Ultra with turning off some stuff
The single 275 is doing fine at high quality at your resolution
but the problem is :-
Note that the GeForce GTX 400- and 500-series cards are DirectX 11-capable, and the 200-series boards are limited to DirectX 10. This is an important distinction because, even though the 200s throw up some reasonable performance numbers (especially the GTX 295), they’re not doing as much work. The game automatically dials Terrain Quality down from High to Low, yielding some pretty nasty artifacts as shadows interact with the environment.
Hopefully Nvidia can fix this in its drivers. For now, those older Nvidia cards don't do Battlefield 3 any favors; stick to the newer stuff (or go the AMD route—incidentally, AMD’s DX 10 cards don’t have this problem).
So i guess you won't get any benefit from playing at high or ultra as there will be some artifacts.
You won't experience a frame buffer issue with that VRAM size as long as you stick with that resolution, but you'll get really disappointed if you play the new GTA as it much depends on the VRAM.
For that said a dedicated Physx card, you will only have a benefit in games that support Physx, besides any strong high end GPU from nVidia is will handle any game with it's Physx engine. Unlikely, a dedicated Physx card can bottleneck the main GPU if the PhysX effects are partially running on CPU.