just put out the few extra bucks and get the 4870 you won't be sorry. I have the 1gb model it plays pretty much everything on my 22'' 1680 by 1050 im sure it can do higher just as well im planning on getting new LCDs soon 24 or 28 inch depending on pricing in April
The size of your display is a concern. Check the VGA Charts here at Tom's for great comparisons. I am running a 4850 on my current 19" LCD and have no problems whatsoever. At some point more expensive/powerful video cards only pay off at resolutions above 1600X1200.