I recently purchased a GTX-460 and expected it to be able to supply 3 monitors. However, this does not seem to be the case (that's what you get for trusting a random store clerk instead of doing your own research I guess). However, I have also purchased the screens already, so just settling for dual head is no option either. Thus, what's the cheapest way for getting my three heads to work? My PSU delivers 530 watts and an SLI capable mainboard is at hand.
Do you want a different view on each screen? If so, you'll need a second video card or a new video card that supports 3 monitors (ATI Eyefinity line for example).
If you want 3 monitors but you want 2 different views (1 + 2 cloned), you can always get a DVI splitter (make sure that your card has Dual Link DVI ports) and split one of the signals. Or another type of splitter based on the connections that you are using.
You need to find a card that uses the same driver (i.e. two identical video cards). I use a Nvidia 3700 Quadro and a NVS 295 Quadro - same exact driver. This was a requirement in Vista and I think it is the same in Windows 7. XP might have let you use different drivers - not sure. It is best to use the same driver though.