If you can get a 5850 for $165, do it. It's a great card for the money.
Personally, I liked the 5850 so much, I got 3 of them - though I wish I paid $165 for each of them. I prefer the Gigabyte OC edition because of the cooling solution on them. They run 10C cooler than my HIS standard edition and are clocked a little faster, but like I said $165 is a great deal on that thing. Although, the Diamond is the one to which you're referring Wiinoppongamer?
When I got my second 5850, I chose the Diamond. I had to return the Diamond because it failed to function. I took it back and got the Gigabyte. I see there are quite a few rebate options out there now through newegg.
The cost of all the other 5850s are still around $200 and above with $20-$30 in rebates. The Diamond used to be one of the higher priced 5850s running $279 everywhere 3 months ago. I wonder why the Diamond dropped that much? Even at that, it's still a good price if you get one that works. For me, when I have choices and am forking over a nice chunk of change, I tend not to give a brand a second chance. That's just me.
Parry93 - If you have the coin for the 5850, choose the brand you trust and go with it. They all are pretty much built to the same spec, though like I said above, you can find some factory OC editions.