The only real difference between manufacturers is warranty and cooler. XFX has a really really good warranty (lifetime for you, and if you sell it, lifetime for the buyer. I would suggest either of these:XFX Radeon 5870
This one's cheap and its XFX. Only $410Gigabyte Radeon 5870
This one's got a massive stock overclock, running at 950 core. Also great double fan cooler. But, $90 more.
The Gigabyte one does have a great stock oc, but the other card could probably achieve the same. I would go for the XFX.