Well, if you are going to get one, get one from WACOM.
I have a 6X8 Platinum and spent over a half a grand for it, about 3-4 years ago.
Still works like the day I bought it, even though ever since I picked up my WoW addiction I haven't really used it much lately..
Get one you think you'll enjoy the most, by that I mean what size..
With this 6x8 I feel that I can only draw on a canvas 1024x768 and no larger (I prefer 480-640), I know I -can- but I just cant seem to draw high resolution stuff.
Do you like to make large pen strokes as you sketch? Do you prefer smaller ones?
Since you'll be usually just using the tablet for brushes in PS, try and get an older model like an intous2 (Which I have) for a cheaper price.