I have 23' 1920x1080 and another 20' 1600x900 monitor. I also have a Radeon 4000 series card from my old dell. Could I use this to run the 20' monitor to do other things like web browse, while I game on the 23'? I have a Radeon 5970 for my main card.
You should connect 2 monitors on the 5970. It's such a strong card. And it would be a better setup.
The reason being is that both are AMD and both will struggle which driver's to use. You can do an Nvidia and AMD card together for separate monitor's because windows 7 allows you to use different video drivers at the same time.
thats backwards from the normal consensus. you can do AMD cards (or nvidia) together because the driver is the same... although you'd be stuck on 12.4 since the dx10 cards were just dropped. Its generally NOT recommended to do an nvidia and AMD card together because of driver issues. technically it should work but its a hell of a lot easier to just use cards from the same manufacturer.
That being said there is no reason not to run them both off the 5970, and its the better way to do it. the 2nd display sitting at the desktop is not going to affect FPS at all. And you can't webbrowse without alt tabbing out of the game anyway. all though you could leave a page up to reference etc.