I want to get a <whatever>, which manufacturer should I choose?
Well generally speaking there are a few ways at looking at this:
* Since you're asking then odds are you wont notice the difference in any of the manufacturers ... so get the cheapest one!
* Certain manufacturers tend to overclock their parts and so compared to the stock models, you will get a bit more performance out of them; the flipside is that you'll be paying a price premium. Some examples of such companies are ASUS, BFG, HIS, Sapphire and XFX. Not only will there be a difference in the performance, often there will be a thermal/acoustic difference as special aftermarket coolers are often used (see HIS/Sapphire for clear-as-day examples)
* Certain manufacturers tend to have lower-than-average prices: some examples of such are eVGA, MSI, Powercolor and Sapphire. Sometimes there's a catch but sometimes (probably more often than not) you're just getting a better deal without any strings attatched
* Some makers are generally known for doing really wild stuff with their cards: ASUS, eVGA, Gigabyte, HIS/Sapphire/Powercolor etc are some examples
But yeah, for the most part, it wont really matter all that much between makers.