I think quad cpu's are not the best choice for most trading platforms, because, like several people already said, most current trading software is not multi threaded. Therefore, the current fastest dual core (e6850 @ 3ghz) will give you better performance than the current fastest quad core (Q6700 @ 2.66 ghz) and is also cheaper.
The 5% cpu usage on a quad core is an optical illusion by the way: If the software runs in one thread and occupies 20% of one core, the cpu usage will show as 5%, but in reality, it's the 20% that matters, if your software is single threaded that is.
So if your system shows 25% usage, you might already be in trouble (it might mean that one core is maxed out and slowing down your system. Sure, you still have 3 cores that would love to help out, but can't do anything because the thread can't be split up).
I'm currently on the Sterling platform. All the L2's, T&S etc run in the same thread. However, the charts run each in a seperate thread, so that's why dual cores will perform way better with Sterling than a single core system, but I don't see any reason to buy a quad core, unless Sterling would multithread their base process, but I don't see this happening ...
