A core 2 duo would be a big upgrade as well, and a lot cheaper. If another component on your computer bottlenecks, you might not see much additional benefit from a quad vs a dual core. What is the rest of your system, and it's use?
Are you sure that your motherboard supports an upgrade to the Q8400? It might be LGA 755 (Socket T) but that doesn't mean that it will support such an upgrade at all. In other words just because a CPU can physically fit in a socket doesn't mean that it's supported or will work. One ideally should know the supported CPU's on the manufacturers website for that particular model.
YES! I'm running a Q8300 right now and it serves me well on just about everything.
What are you using the PC for?
Also PC specs would be nice - It may be you end up chucking money at it for no reason...