Standard DVI can do 1920x1200 with no problem. You only need Dual Link if you want 2560x1600 and up.
Also, Andy, the regular mini-dvi to dvi adapter will work up to 1920x1200. The Apple rep is just trying to get you to buy the stupid dual link one, which is overpriced from Apple. I asked them the same thing when I bought my first mac and was considering a macbook, and they flat out lied to me saying it will only support 1280x800 on the mini-dvi. Thats a crock, check the forums here, on some of the older "Post Your Mac" threads, there are macbooks driving 24" screens.. I have a 1440x900 running on Mini-DP to DVI (standard, not dual link, and from monoprice) and it has no problems.
EDIT: Also, if you get an adapter, get it from Monoprice. It's way cheaper and imo, better bang for the buck. I havent had any problems with my stuff from them. Quick shipping and cheap.