In the actual numbers, there isn't much to see. In reality, there are some changes, depending on your budget that might be worthwhile: improved battery life/power consumption/better graphics card, etc. For me, however, the real-world performance is great on MBPs even as far back as the Core Duos. And I was relatively happy with older generation PC hardware before that. Upgrading in close succession always carries a premium.
All that to say, I really wouldn't mess with it, especially if your computer's been reliable for you and you're happy with how it's running.