First, I don't know what the two of you have a problem with, but the OPs thread is NOT the place to air out your problems. Go the F away, no body cares about your issues.
So the question is, E8400 or 710/720? Lets start looking at benchies.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3512&p=8
Look at the L4D benchmark seeing as you play steam. 8400 gets 117, the 720 gets 112, and the 710 gets 107. The 8400 gets 10 extra FPS, or about 10% faster then the 710. All chips are over 60FPS, its not like either gets you that much more. The other game reviews from this site show the same 10% faster, except for Farcry 2 which is only 2FPS faster.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3512&p=5
If you do any kind of encoding/video work, the 710/720 is faster with the exception of encoding to WMV. Windows media encoder 9 only recognizes even numbers of cores, so the extra core that the 710/720 has isn't used. Using other programs however show the 710/720 is faster then the 8400.
So what does all this mean? The 8400 is a bit faster for gaming, and a bit slower in other tasks. None of these chips are so much better then the others that one MUST purchase it above all others. I would get which ever is cheaper, spending your money on better GPUs or drives.