$300 will get you a 1920 x 1200 or 1920 x 1080 LCD. The HD 5970 is basically overkill for that resolution unless you are trying to max out Crysis in which case i would wait another 5 years for a video card that can give very good frame rates with max graphics quality.
My only experience with a monitor in that price range is the Asus VK246H see following review. The Asus VW246H is the same as the VK, but no webcam.
There is a $999 2560x1440 monitor out now, toms did a quick review on it recently.
It isn't the size, but the resolution. Generally when people refer 30" monitor, they are actually meaning to say 2560x1600, the common large resolution. Those are often 1300+.
That card could easily run 3x 1920x1200 (or 1080) monitors in eyefinity (a flagship feature for the 5xxx cards). 3 of those for a couple hundred a pop would give you the real estate to work your card, heh.
Well I pulled the trigger on a Dell 3008WFP and blew my budget, oh well.
Having a 2560x1600 monitor is one of a few configurations that can utilize and justify the power of a 5970, I wouldn't consider that a bad decision at all (and wish I had the computer budget for that sort of real estate and horsepower, indeed. A little bit of envy is good for motivation, afterall. )