Best CPU for D/P5K Motherboard

I got out an old 2007 PC. It belonged to a friend but he uses a better one now.
Anyway, the CPU is pretty crappy, and I can get an upgrade around July.

What is the most powerful CPU I could get for a D/P5K Motherboard?

Hi,
this website gives a list of possible CPU's and related performance. You might even struggle to find one? Check it out anyway:
Asus P5K SE Motherboard: A List of CPU That Fit Asus P5K SE

Thanks. Looks useful...