New rig - go 3570k plus z77 and play with Virtu MVP at 4.5-4.6ghz.
Only reason to go older:
1) z68 and P67 mobos new and used are cheaper
2) You can find someone to sell to you a "proven clocker" that can do 4.7-4.8ghz.
A lot of 2500k's are very mediocre at hit a reasonable voltage ceiling at 4.4-4.6ghz.
3570k's easily clock to 4.5-4.6ghz it'll be the temps that stop you.
And a 3570k will perform like a 2500k that is 200-300mhz higher that it is.
i7-3770k 4.8ghz 1.21-1.24v Delidded 3231B415
i7-2600k 4.8ghz L041C108 1.32v PLL off Venomous-X Push/Pull
Asus P8P67 Vanilla
Samsung 30nm MV-3V4G3D/US 2x4GB @ 9-10-10-28 1T DDR3 2133 1.6v
Diamond HD 7970
CM 690 II Advanced