Asus p5 kpl am se upgrade

What you have on that motherboard ?
Cpu upgrade , you can upgrade to an quad core for 775 socket Core 2 Quad Q9650 this is the biggest cpu upgrade.
The maximum memory what your mobo support is just 4Gb. ( 4 x240-pin DIMM, Max. 4 GB, DDR2 1066(O.C.)/800/667 Memory)