I think you have to stick with cards that use the same video driver though. So ATI and ATI or Nvidia and Nvidia. I believe there are ways to work around this.
4670's are good multimedia cards, if you don't plan on doing gaming. You could toss in 2 of those.
Yeah I will have to second this advice, two 4670's will do plenty well for 2 DVI monitors in extended and a TV at 1920x1080. Make sure you get the same company, brand, and model versions. They should be fine. Does your motherboard have 2 PCI-E 16x Slots?