How much does the brand change?

[Monetizer]

So I have found a really cheap 5850 but it is not a brand I know (amaze-tech) and I was wondering how much will the performance differ between different brands?

[notaek]

You should be fine.  The only thing that differs between the manufacturers when it comes to reference design cards is the support.  The warranty of most 5xxx series cards is coming to an end anyways, so it won't matter how good or bad their customer service is.