How do the benches compare between the two (e.g. toms, etc, etc) ? i.e. does the ultra push framerates significantly higher than the 670 (stock?).
Then, only you can decide. Bear in mind that cuda is significantly better, in most cases, with the 580 (not fully implemented in the 670) - if that's important to you, otherwise for games, check the benchmarks from multiple sites.
If your gaming with mulitple monitors 3 is the only way to go as with 2 monitors the bezel is in the middle of the 2 screens(not good). The gtx 670 will run the newer games at 5760x1080 but you will have to turn graphics settings down to medium and possibly anti-aliasing off depending on the game, in order to receive good framerates.
If your using multiple screen just for work/surf 2 will be fine and the multitasking on them is great and i find it hard to work on just 1 monitor now.
looks like the 670 is the best bet here. im looking to run 2-3 monitors at the most. working with AutoDesk programs etc, also lots of gaming.
Ahhhh! if you are using GPUcompute in autodesk apps, get the 580!!! nvidia crippled the 600 series computational power especially in double precision so their quadro series would be more relavant. Look it up! the 580 DP is about 1/8 its SP, whereas the 600 series DP is about 1/32 that of its SP. Slow! amds solution in openCL even kicks the 600 series ass!