Motherboard with x16 slot but x4 bandwidth - Minimum Video card to max it out?

I have been running this mobo with my 8800gt 512 card, and my old 8800gts 320 card for a while now, but I was thinking, that since my second slot is only x4 bandwidth, that Im starving my 8800gts for bandwidth. I had taken this card out of my secondary gaming pc when I got 2 new monitors (well not new, but I was using them at my office, and switched jobs, and the new job actually provides decent monitors...)

So I have a PC without a video card, and one with a video card thats overkill. I want to replace the card in my x4 bandwith slot, but I don't want to go too cheap.

I was wondering how I might find out how much bandwidth an x4 slot can muster, and whether or not a cheap low end video card will be enough. I was looking at a cheap 8400gs. Will this be enough to drive 2 monitors? right now the monitors hooked up to my "starved" card can run 4 wow windows fairly well, but each can barely do 20fps, where on the monitor hooked to the full bandwitdh slot can do 40 fps+ easily.