I have a modest computer, with one 8800gtx and 4G of RAM. I run a 30" at 25x16 and a 20" at 16x12. I run my main toon at 25x16 on the 30", and the 4 clones at 8x6 on the 20". I fps cap all the slaves at 15. I run windows xp and use dual-view, not horizontal span.

Given all this, I get (at 2560x1600 resolution remember) 40-60fps on my main.

The main things I did to make this happen:
  • I use one directory for my main toon, and a second directory for all the slaves which is NTFS junctioned off the main directory - search around here for junction, you'll find lots of info. This is key to getting good performance, especially when zoning or loading new textures. I do not recommend using multiple directories. I especially do not recommend running different directories across multiple disks. Yes, I know it sounds like it should help - it doesn't. It made it slower (for me anyway). I blame (praise??) per-disk caching.
  • The configuration on the slaves has absolutely every graphical option to its minimum setting. Make sure you don't forgetting to disable anti-aliasing with your video mode selection.
  • Keep addons with graphical components to an absolute minimum on clones. I leave my clones at the stock UI, with just a very few mods that I need to help me manage the multiboxing (none of which are graphically intensive).
  • BY FAR the biggest help to my frame rate was disabling vsync across all windows. Yes, this does induce tearing, and believe me, you haven't seen tearing until you see it at 30". It took me some time to get over, but I much prefer a smoother frame rate to the lack of tearing. It's a trade-off you'll have to assess yourself.
Those are the steps I took, and I'm pretty happy with the results. I may upgrade at some point because I'd like to lock my main at 60fps, but considering the resolution I'm running, it's not bad at all.