Would have to agree with Wardy and PriyeshDesign. Wacom are perceived to be the market leaders. There are alternative brands out there but to be honest I immediately went wacom when making a decision to buy one.
The Intuos Pro medium is one I bought last year and really enjoy using it. I would consider buying via Amazon as I picked mine up for £270 originally, think it is £230 now, as opposed to £299 on Wacoms site. You could opt for the large
and still have change from your budget. I use mine on a 24" monitor and it scales perfectly.
The Cintiq range are possibly the best but again can be overkill depending what type of work you are doing. With your budget you might have to put a little more in to purchase the entry level size.