I'm currently running a E8400 with a P5N-D Mobo (SLI ready) etc...just wondering if I can simply swap chips for that new i7 - 975? and would I notice a big difference?
No, there not a noticeable difference til you run apps that are multithreaded, which most are not. When apps catch up there will be a huge difference as there is on the multi threaded ones already.