I'm pretty sure you can run two unmatched cards... only I would worry a little about driver conflicts (especially if one is ATI and one is Nvidia). The problem is deciding what slot to use and how much to spend. I would say make it two cards from the same GPU manufacturer, and spend the minimum amount possible on the second card (find a plain old PCI card if you can, then you wouldn't have to change motherboards).
If your current card is an Nvidia one... here's a possibility without having to change your mobo -
The only other thing to be concerned with is power... if you get a second card that requires a power plug, you should make sure your current power supply can handle the load.
I think the biggest hurdle you're going to run into overall is the software/drivers. With three monitors, I'm guessing that with either ATI or Nvidia you're going to run into some really silly software glitches and strangeness. It has been a long time since I've played with multi-monitor setups, though...so things may have improved.