Pipe dream? ...no. Undesirable? Yes. Those 8800's are about 5 years old (if not more) and they came with 768MB of video RAM max (may have been a 1GB version but it was super rare). With those old cards you'll be forced to use DX9 and running 5 clients will take at least 500MB on the lowest of low video settings.

I have no idea what type of frame rates you can expect to get running 5 clients on an 8800 but you can't take advantage of SLI. If you had 2 monitors you could attach one video card to each monitor to try and help with GPU load but, that dual-core CPU is going to limit you the most. I'm going to guess it's going to be seeing 65%+ load in an empty zone (that's just a guess).

I hate to say it but, I doubt you'll be able to walk through a major city or do a large scale battleground with all 5 clients at the same time using your current setup. If we were back in WotLK it'd be different but, Cataclysm brought an upgrade to the game engine/textures and MoP will probably do the same.