I've never tried running two discrete graphics cards that weren't using the same driver....can you mix/match Nvidia and ATI and have them both work?
That said, instead of bogging the system down with two cards (especially with an old 9800GT that idles at 35W), why not just get the appropriate GTX 6xx card and be done? What's your gaming resolution and games you want to play? If at 1080p, you can just get a GTX 660 (next week, probably $225-250) or GTX 660 TI (~$300).
If you move back to 1 dGPU, then you don't need a high end ATX mobo to support multiple GPUs. If you want FanXpert2, then get the z77-V. If you don't want to turn the fans off, but still be able to set profiles, then get any of the Z77-V LE, LK, etc that have the features you want. Or, look at the mATX versions if you want to aim for a much smaller case.
PSU: Seasonic X-560, Kingwin LZP-550.
Thanks Steve. I already have the GPU (and the ssd's) they are coming from the system being replaced. You can use the AMD and Nvidia card together. It is a bit of a hack but it works fine for me. It is not absolutely necessary, but I like the little extras it gives me in some games. FYI http://www.ngohq.com/graphic-cards/22136-how-to-fix-hybrid-physx-with-latest-physx-and-geforce-285-solved.html
If I can't use the nividia card for whatever reason I would not lose any sleep over it. Thanks for the PSU suggestions. Any feeling on a case? I have plenty of room to fit a case, it just seems that because I don't have many add-in's all the space seems wasted. My p183 has/had 1 hdd and 1 optical drive it it and lots of spare room. Thanks.