I have two 20” LCDs running at 1,600×1,200 resolution in a side-by-side configuration (hence why I was able to capture the really wide screen shot).
Someone running a single 30 inch Apple Cinema Display is likely running at 2,560×1,600 resolution.
Point is, who are you setting this large, non-repeating background up for? If all possible users or any possible configuration, then you’ll need to get crazy. How about that 30” display turned sideways? You’d want an image at least 2,560 pixels in height then.
Or what if someone has a 4 screen rig comprised of those 30” Apple flat screens, two on top and two on the bottom? That’d be 1,600 + 1,600 = 3,200 pixel image height then.
Personally, I suggest just duplicating Trey Ratcliff’s approach… use an image 1,200-ish x 800 and be done with it.