Newegg has a Q8400 @2.66 cheaper than Q8300 2.55 by $4. Any reason except sales volume for this?
More aboutcompare intel quad core
The Q8400 is a better buy as it supports Intel VT-x (required to run Windows XP mode in Windows 7 or Hyper-V in Windows 2008 Server) and it costs less. Intel has discontinued the Q8300 and not all revisions support Intel VT-x.