If you plan on doing gaming with that quad you are gonna want to overclock it to atleast 3 ghz or higher if you can.
To answer your question better you should list what you plan on using the system for.
Here is a comparison benchmark for the two processors the Q8400 will multitask better and encode faster but for gaming the E8400 will be much better in most games with its higher clock the E8400 will OC really good to 4ghz or higher with little voltage increase and a good cooler.
To answers yalls questions... its not a new build, it was kind of an upgrade, but more of a replacement... the other specs are:
3GB of ram
320gb hd (mid range in speed, don't remember the specifics)
reading the comparison really helped. It's actually my brothers computer and I'm barrowing it. It fell off the table the other day, and all the PCI cards popped out, the processor fan broke off, and the Hard Drive started making this horriable groaning sound. Luckily it was off when it happend. The processor fan would no longer snap in place, and I pulled a dumb *** move and put the computer side-ways and just rested the processor fan on top of the processor. It booted up fine, and I started trying to copy an image of the hard drive real fast onto a spare, however it self-shut off before I could do it. I tried turning it on again but it shut off after 5-10 seconds of booting. I figured the processor was burned up, and I knew the HD was failling so I bought both a new HD and a new processor. With hopes of still copying over my HD, I installed the new processor and tried to boot, however it wouldn't work, so I unplugged the old HD and then it worked fine. So I'm now thinking I probably bought the new processor for no ******* reason, and just needed to replace the HD & buy a new fan (the old one was a 3ghz dual core too)... but oh well!