Need some help in over-clocking Q8400

hello all,
Just purchased a new rig after my last one got stolen. Its not that awesome i know. but what i need to know is that:

1> How much can i OVERCLOCK(had written upgrade making it a silly question)EDIT) and what method would you recommend?
2> How the hell do you increase the hardisk score beyond 5.9? Really annoying when everything else is 7.4!


CPU: Q8400 quad
RAM: XMS2 @800 2 sticks (was considering installing a fan on top of them as well later on)
Mobo: Asus P5QPL (will install a fan on top of southbridge and northbridge as well later on)
CPU Cooling : Corsair Liquid Cooling Kit Hydro H50 for the cpu
Native Resolution: 1080p
GPU: HIS 4890 icooler x4
Casing is : CM Elite 334 NV edition....

Currently i got only one fan in the front for taking air in the rig...
No place left for installing a new fan, any recommendations? WIll upgrade to cm 690 NV Edition later



Is it possible to get the speeds to 4.0Ghz? What would i require?

Also the motherboards i can purchase are the ones on this website ONLY :

http://www.galaxy.pk/forms/motherboard.htm

THANKS ALL