I have read that having >4gb of ram can cause problems, try taking out half your ram and see if that fixes it. If it does you should be able to put that half back in after the install is done.

I have pretty much the same setup as you only I have a Q8400 and only 4gb of ram and I can boot off the OS X dvd fine using iBoot supported (Haven't installed it yet because I only have the setup DVD for my MacBook and it wont let me install on my desktop because it doesn't see it as a MacBook).