Depends on your monitor, I guess. If you're planning on making desktop backgrounds for others to download, you should probably make a few versions for different monitor sizes and aspect ratios.
The most common ratios/sizes on the Mac are the same as on PCs. Specifically:
4:3 1024x768, 1280x960 1600x1200 (mostly used on CRTs)
5:4 1280x1024 (mostly used on LCDs)
8:5 1680x1050, 1920x1200, 2560x1600 (used on many large widescreen LCDs, including Apple's)
3:2 1440x960 (currently used on the 15-inch PowerBook, and I assume equivalent PCs)
Again, same as on PCs.