Multiple video cards on Windows 7

Theoretically yes. Though running graphics cards from two different manufacturers will more than likely be a problem as you'll need to install two sets of graphics drivers, and these won't sit well together.

In fact you might even have problems if you install two different model cards from the same manufacturer - but only if the cards require different drivers.

I'd buy another card of the same make and model and use that. If you do then you should be able to use 4 monitors.

Disclaimer - I've not actually tried this, just extrapolating from my knowledge of graphics cards and display drivers gained in over a decade of working in computer graphics.

As long as those graphics cards have WDDM 1.1 compatible display drivers you shouldn't have a problem in Windows 7 (in theory that is). Previous versions of Windows e.g. Vista, required you use graphics cards from the same manufacturer with the same driver.

If you can make do with 3 monitors you could get a Radeon HD 5770 which offers the same or better performance as the Radeon HD 4850. SLI and CrossFire are irrelevant in both cases.