Re: ATI 4800 series overclocking
AMD GPU Tool allows over 850 on the core. Just make sure you can cool the card, because you'll lose the hotfix profile for the fan if you use it to overclock.
i7-2700k @ 5.0 GHz
Nvidia GeForce 570 2.5GB Tri-SLI
Asus P67 WS Revolution (Tri-SLI)
OCZ Vertex SSD x 4 (Raid 5)
G.Skill 8GB DDR3 @ 1600MHz
PC Power & Cooling 950W PSU