I don't think it will be possible to use a 6990 in a 32bit system mainly for the fact that you can't have more VRAM than DRAM and 32bit only supports 3.25gb while the 6990 has 4gb, hopefully someone can confirm if it's true.
I looked at AMD's website and they do have a Win 7/Vista 32 bit Driver for the 6990 so I guess it's possible. Why don't you have 64 bit? I mean it's a $700 graphics card with 3 monitors, you should have at least 8gb if you are running a rig that expensive. It will slow down and again I think there will be an issue with you having more VRAM than RAM. I'd do a fresh 64 bit install when you get your card so it will run at peak performance. Also your CPU should be highly clocked if you want to avoid any bottlenecking issues, around 3.6ghz at least.