[Kibblre]

Decided to check out that 3-way SLI setup and ran into this:

"Before we get into it though, we have to laugh at our power supply situation. When we started testing our Tri SLI setup with a 1000 Watt Zalman PSU, we ran into some problems."

Also, there only seemed to be a ~3-5fps difference between running 2 and 3 of those cards from the benchmarks I've seen. It seems like you should just take one out and use 2 of the 280s with a 1000 watt PSU.

Finally, overclocking a processor is pretty easy but overclocking a GPU can take a lot of time and messing with (about 2-3 hours if you're doing it the "right" way), but it probably wouldn't be worth overclocking a 280.

EDIT: You should be fine with a mid size tower. I have a similar sized mobo and I have a good amount of room in my case. I can get a picture if you'd like.