wa1 is right, if you have a 2.0 pci-e motherboard, then don't worry about it. If you have a 1.0 pci-e motherboard it may be better to go with a 2.0 card.
Brands really don't matter, the only options they really change are the cooler, heatsink, warrety, and then overclocking (which is something you can usually do on your own for free, and better). There are some brands that are better then others, XFX, Sapphire, ASUS, and HIS come to mind.
i was going to go for the msi r5859 because it had the r5770 hawks cooler which is apparently awesome, but then i read a review that said the twin frozr 2 was perfect for the 5770 but not for the 5850, and that the r5850 had problems, after reading many reviews 3 brands stood out the most, sapphire, asus and xfx. i only have access 2 a gigabyte 5850, as i have my own business, i buy from rectron and sell but rectron only stock the gigabyte, any of u know any problems with the gigabytr 5850?