Depends how much you really want to push it and how much you enjoy testing and tweaking BIOS settings? DIGI+ VRM has a ton of UEFI options around this and each phases provides more power than the vanilla P8P67 series. You can see most of the options covered by BT here: http://www.bit-tech.net/hardware/cpus/2011/01/07/how-to-overclock-the-intel-core-i5-2500k/3 (old UEFI version now), and if you have any other specific questions on the options I can answer them. If you're interested in really pushing the limits of your K series, the DIGI+ is the way to go, however BT got 4.9GHz from the vanilla board anyway, so I doubt most people happy with hitting say, 4.5GHz, would notice. That said I haven't use the -M tbh only the ATX boards. I'll see if I can rustle up a board and CPU from the product manager, but it might take a while, or look in the Silverstone FT03 thread on here - several people are using them there.
Damn it, I want one now! Thanks all for the advice, I'll pass it on to him (and maybe persuade him to join the forum if I can wrench him away from the (admittedly excellent) Rock Paper Shotgun forums while I'm at it)
I got a Asus P8P67 Intel P67 'B3 revision' When i got my PC from Build Your Box/ and a 2500 CPU and its got good features like good connections and good overclockability(not that ive overclocked mine)
Doesn't the vanilla already have DIGI+? (thanks thefriscokid btw ) Looks to me like the only difference between the vanilla and the Pro is SLI support, a PCI-E x16 and an ESATA.