I got one of those "Xtreme" edition Sapphire 5830s from NewEgg (one of the $109.99 dealies). I think in order for them to be "Xtreme" they have to have really good quality cores to handle the slightly higher clock rate. As a corollary this means they can be pushed to slightly higher OC levels as well. At least I *think* that's what "Xtreme" means. Or it could just all be marketing hype.
The only thing that makes the Sapphire "Xtreme" is that it has a better cooling system and some voltage tweaks that will let you overclock with better stability. Other than that, everything is EXACTLY the same as the non-Xtreme version. Same hardware, same clock speeds etc
I wouldn't go for the Sapphire Xtreme (again)... I have one, and my ASUS DirectCU can clock MUCH better, and stay much cooler than the Sapphire Xtreme.
Also, the ASUS has a much quieter fan than the Sapphire to the point where I'm not exaggerating when I say that my brand new Sapphire is twice as loud as my 10 month old ASUS.
And no, the heat isn't because it's above the ASUS because I swapped them and was getting the same high temps.
Edit: Oh yeah, what Xtoro said too. I'm using AMD GPU Clock Tool to set the MHz, and it automatically pegs the voltage at 1.025 or something like that. I have no control over the voltage with that tool.
Try using TRIXX, works for me on both cards quite well.