(2) GTX 680s in SLI VS. (2) AMD 7970's in Crossfire

i would go with another gtx 660 or a pair of gtx 680's. like you physx does matter to me. yes you can use hacked drivers to get it to work with an amd card gpu (visuals) and dedicate nvidia for physx gpu but again HACKED drivers...i rather use certified or beta drivers (beta only when i have to)...sli has less driver issues then crossfire so that's also a kill point for me and AMD. And of course AMD 7000 series has frame latency issues...high frame rates can still look choppy. now some fixes have been added to drivers and more are supposedly coming. but in the end for me there are just to many negatives with going amd to make up for the price/performance ratio advantage amd has.

also...you get a pair of gtx 680's if you have another PCIe slot to spare you can dedicate you gtx 660 to physx.

Two 680's is your best bet.

The 680s are pointless. A 680 is only 5% faster than a 670, but it costs 25-30% more.

Buy two 670s for $350 each, and you'll run circles up with the Titan and the 690.

A couple notess:
- The AMD PhysX hack no longer works with any of the latest Nvidia drivers, so newer games will not be supported. The last drivers that worked were something like a year ago.
- A 7970 will not "run circles around a GTX 680". Perhaps you meant a "GHz" edition? Even then, the results will depend on the game. Tomb Raider 3, for example, was shown to now run slightly better on the 680 than the 7970 GHz with the latest drivers.
http://www.hardocp.com/article/2013/03/20/tomb_raider_video_card_performance_iq_review#.UUvfyXChDFI

Here's a nice review comparing SLI and Crossfire in terms of frame latencies:
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/60166-nvidia-gtx-titan-vs-sli-crossfire.html

Here's a review of GTX 660 Ti's in SLI vs. a GTX 690, which should be comparable to a pair of GTX 680's in SLI:
http://www.xbitlabs.com/articles/graphics/display/geforce-gtx-660-ti-sli.html

If it were me, I would go for the second GTX 660 Ti, which should be everything you need for a normal 1080 or 1200 monitor resolution.

"atomicWAR" is correct, with the latest drivers the 7970 will NOT run circles around a Gtx 680. A 680 will have comparable and occasionally superior frame rates in many recent games (although bear in mind that any hierarchy can be subject to change according to the resolution and further driver updates). At present, a standard 680 will generally outperform both 7970 and 7970 Ghz edition in Crysis 3. The 670 aquits itself very favourably too in comparison. Remember that this could change with further driver updates though.

http://www.bit-tech.net/hardware/2013/03/15/crysis-3-performance-analysis/6

It is often said that the 680 has more headroom and potential for manual overclocking. Furthermore, apart from the obvious PhysX, FXAA, TXAA and 3D vision bells and whistles (which admittedly not everyone cares about) and in addition to the power efficiency and better temps, the 680s appear to be on top with frame TIMES which are often overlooked or unmeasured in reviews. You want smooth? The 680 is generally a better bet.

http://uk.hardware.info/reviews/3991/amd-radeon-hd-7970-ghz-edition-vs-nvidia-geforce-gtx-680-frametimes-review

That being said, in terms of pure performance there is an argument to be made that the 7970 is a better all-round card; "zander 1983"'s claim can be said to be correct provided you are only talking about games that rely heavily on "direct compute". e.g. Sniper Elite V2, Sleeping Dogs etc. In that respect, this gen's AMD cards do generally run rings around Nvidia's (with the sole exception of the Titan). A gtx 500 fermi series will generally give you better performance than a 600 kepler series in such games.

I recently bought a custom build and chose the 680 SLI option simply because I actually like 3D and Phys X. I was originally planning on dual factory overclocked 670s as the builder was offering them for no extra cost over stock clocked and they have a comparable performance to a stock clock 680 for a lower price... but then I managed to get the builder to upgrade these to factory o/c'd 680s for comparatively minimal extra cost.

So to answer your questions:

1. IMO if you are not bothered about 3D vision, TXAA and FXAA then bite the PhysX bullet and go for the as its 7970 its cheaper (unless YOU pay the electricity bill and game for several hours each day) and is far stronger for Direct Compute games. You may be able to find a reasonably priced Ghz edition if you shop around.

2. However, if the above IS important to you then go with the 670 Sli as suggested by "Dark Sable". Best price/performance ratio. The jump in performance from 660ti to 670 is far greater than the increase yielded by 670 to 680.

3. To my knowledge this would not be possible without a hack and even then each new Nvidia driver update would likely have a counter measure rendering the current hack redundant until an update or new one came along from the hacker's side.

Look at some of this advice... While it's true that 670 offers pretty good value compared to the 680, there's simply no point upgrading from 660 Ti to a 670, whether you get single card or SLI, PERIOD. And while we're in the business of recommending products by value, 660 Ti happens to offer even more bang for your buck than a 670 does.

If you're gaming at 1080p then I suppose a second 660 Ti is all you should get for now. This generation of cards is already a year old and your current one is this gen so don't be hasty. If I were you I'd just get another 660 Ti and perhaps wait for next gen if you wanted to SLI something with a bit more bang. (Chances are 2x 660 Ti SLI will still give a single high-end next gen GPU a run for it's money, but that's just speculation.)

-Respectfully disagree given the reviews I have read but they were mostly hard copy publications so can't be bothered to look for them now. The OP will just have to trawl and compare the review sites for the 660ti vs 670 conundrum ;-)