I'm using 4 displays hooked up to 1 nvidia quadro graphics card, 19 inch monitors (1280x1024), 3 of them oriented vertically so i can get long pages of code / webpages on them easily. Horizontally oriented one is technically the primary (and leftmost) for IRC/comms and the occasional game of UT. Xinerama and KDE behaves fine, not using compiz or anything fancy tho.
I highly recommend getting multiple displays - you will be more productive. Orienting vertically also means I don't need an enormously wide desk.