I read somewhere that windows only uses one graphics driver at a time. I think this might work for me, but the details are a bit scetchy to me.
Windows Vista only supports 1 video driver at any time, XP and Win7 both support multiple video drivers. However the 7800's and the 295 are in the same driver file, so this would not prove to be a problem for you either way.
Also I dont know if anyone can recommened if i should mix and match size/shape monitors for a 3 screen setup. is it recommened to use a square middle with widescreen sides.. or keep em all the same?
Personally I prefer to use the same for all displays, however this is entirely up to you and what you feel is a suitable setup.
also if using 3 video cards (2x7800gtx, 1x295gtx) wont work do recommend I pick up a eyefinity or matrox 3 monitor gadget.
Unless you would be gaming in windowed mode all the time, I dont see the 7800 SLI setup having any impact in performance, let me explain why...
Method 1: Matrox TH2G-DE / MView
When using these devices, they connect to a single gfx card and produce a large virtual resoltuion which is then split into 3 for each of the monitors.
In this case as the device connect to a single gfx card, it is this single gfx card alone that will do all the rendering for all 3 screens, your 7800's would not even be used.
Limitations:- (I dont own one but this is from what I recall about them)
@ - Matrox - No support for DX11.
@ - Matrox - Limited support for DX10 - some titles work when in windowed mode.
@ - Matrox - Limited to 5040x1050@59hz ? (ive not seen or heard of anythign larger than this from a single Matrox device).
@ - Matrox - All displays must be either landscape or portrait, not a mixture.
Method 2: SoftTH
Soft-TH works similar to the above devices, however it is purely software driven, just like the above, it also creates a large virtual resolution rendered on a single gfx card, which it splits into 3 for each monitor by sending the signal over the PCI-E bus, for this to work you do require a minimum of 3 display outputs such as the 3 on your 295 when not in SLI mode, OR with 1 or 2 outputs from your 295 with the 2nd and/or 3rd displays connected to a 2nd gfx card, such as a single 7800, bear in mind however the 7800 will not be rendering any of the game, its simply passing the video frames, so even a very low performance card like a 7400GT PCI-E would work just as well.
This is the setup I use with my main card being a GTX285 and the 2nd card being a low 9600GT.
Limitations:-
@ - SoftTH - No support for DX11.
@ - SoftTH - No support for DX10.
@ - SoftTH - Software driven so has a slight impact on game framerate performance.
@ - SoftTH - Lack of good support, you will find more support here on WSGF than you will from the offcial forums, it would appear the developer has pritty much stopped any futher development of this due to ATi's and Nvidia recent technologys.
Method 3: ATi Eyefinity / Nvidia Surround(coming soon to GTX2xx & GTX4xx cards)
This method is becoming the most favored with a good level of support and ease of use, these card support multi monitor gaming natively and do not require the likes of SoftTH or Matrox TH2G devices, although eyefinity/surround will work in conjunction with SoftTH/Matrox to produce some truely epic display setups.
This of cause would require you purchase new hardware however, unlike SoftTH which is free.
Limitations:-
@ - Matrox - All displays must be either landscape or portrait, not a mixture however this is due to change with an updated driver (unsure if thats released yet or not).
Thats what mostly spring to mind, im sure others here may point out points that I may have missed.