P5ND2-SLi CPU Upgrade Problems

My mate has a ASUS P5ND2-SLi board he bought a few months back a HP Pavilion INTEL P4 516 2.93ghz PC. The motherboard is rubbish so the idea was to buy a new board and pass over the P4. But now he wants to buy the best CPU for the board instead of swapping. After looking on ASUS support site he can go upto P4 Ex ED-840 3.2ghz but I can't find one cheap I saw 1 going for £600.

Anyways I've seen a P4 640 3.2ghz for £120 is it worth the upgrade as the board dosn't support the Core 2 Duo or anything else.

Houw about pentium D 940

http://support.asus.com/cpusupport/cpusupport.aspx?SLanguage=en-us

I've no idea if the results will show but if they don't just sellect the following to bring up the list.

Search CPU Using: Motherboard
Type: 775
Name: P5ND2-SLi

It will then bring up the listed processors it supports unfortuantly the 940 also isn't avaiable. My mate said he saw a Pentium D 840 for £80 but I've no idea if the Pentium D 840 is better than a P4 640.

Which is better from the above the 640 or the PD 840.