For 3D modelling and the like its all about resolution, you want at least 1600x1200 (20" TFT) or 1680x1050 (20" wide)...bear in mind the wideness only really aids toolbars (in my experience), the working area tends to be squarish...so 20" wide will give less working area than a 20" 4:3 screen (although with lots of toolbars onscreen it can decrease the margin). The Dell 3007WFP would be excellent (as would the Apple 30" cinema screen) and a joy to work with, as is dual screens.
I use Solid Edge for technical modelling (engineering application) and I happily get by on a 2405FPW/2001FP dualscreen rig; working on less than the 20" as a lone screen is a pain.
@djohn, 17"/19" screens aren't 4:3, they're 5:4. The limited resolution is just that...limited for this kind of work. You can get by on it, but moving to 1600x1200 really boosts productivity.