Basically ATi are the company who make a chipset, and these companies such as sapphire/club 3d/gigabyte, buy the chipsets from ATi and stick them on their own boards which they design, some of them have special features like Video Input and maybe a Silent Cooler, but all in all, performnce of the cards are pretty similar, but brand wise top brands are Sapphire, Asus, HIS, Gigabyte, so if your looking for an ATi board go for either a Sapphire version or HIS...
Well any card should be able to play back video 2D stuff at that resolution, but if you want one that is media center compatible, i'd go with a radeon x700 AGP as it has DVI and Analogue component output, you can just add a Silent cooler to it, such as the Zalman VF700-CU, total is £100
Sapphire X700 £70
Zalman VF 700-CU VGa Cooler £24
Also alot of people on here have the Gigabyte 6600GT Fanless, which is a bit much at £130, but it silent and is similar in performance if not better than the X700..
Gigabyt 6600GT
Just Make sure you know if you need the PCI-E verison or AGP version...