Mac Pro :: 3rd Screen - Portrait Orientation To Display Printed Pages?

I've got two 20.1" normal aspect monitors right now (Dell 2001FPs), both being driven by the GT120 in my Nehalem Mac Pro. I want to buy a third screen that I can put in portrait orientation to better display printed pages.

1) 24" widescreen aspect.
2) Ability to stand in a portrait orientation (tall, not wide) with the default stand.
3) Ideally a Dell model, since my other two screens are Dells (this would be nice, but is not essential).
4) Hopefully somewhat cheap (~$300).

I see plenty of 24" models on the Dell site, but it is not clear to me which ones can be mounted portrait on the provided stand. I don't care about the screen being able to pivot; I intend to mount it portrait and leave it there.