Which one is the better choice....

I am putting together a computer made up of left over parts for my daughter.
I have a p4 551 and a p4 640, to be mounted on a N18 SLI Abit mobo with a PNY 8600GTS video .... running on 2g's of OCz Gold xtreme...Ram....Which is best in this app. 551 or 640.....and why? .............Thanks

 

i would use the 640 chip so that if at any time you want to use 64bit OS you have the option already

 

Sometimes the 640 P4 has issues with Windows XP SP2, so if that's what you plan to install, you may want to check what revision of the processor is. Revision 7 and lower have problems, the easiest way to fix is to flash the BIOS before installing. Otherwise you may get blue screens as the processor fails to handle the Update.sys file that SP2 installs.

More info here. I've just had to deal with this issue, so I'm still fairly current on it, if you have issues, should you choose the 640 chip.