i love HIS, i wouldnt buy anything else, they always seem to have reliable, and the coolest cards around. if you cant afford that go with a sapphire, stray away from diamond though, and i personally do not take too kindly to xfs cards because they dont seem to stay cool.
All reference design HD 5870s are built the same. Right now the best HD 5870 there is available for o/cing with probably be the Sapphire Radeon HD 5870 Vapor-X - it has a custom cooler which keeps the GPU well, cooler, thus giving you more overclocking headroom. The only other cooler than reference cooler for the HD 5000 series graphics cards would be HIS IceQ versions - but they don't have a HD 5870 version of it.
actually they DO have a cooler for the 5870, it just came out a few days ago. It comes in a 850 (stock) 875, and 900 MHz, here are some links, not all of them are on sale right now because they came out yesterday, but they should all be on newegg.com by the end of the week.