craig - I would look at either Artist ($99) or Home Studio ($49). I bought home studio for a laptop - incredibly powerful and runs well even on an i3. I think Artist may be a better candidate for upgrading. But if you wait for a sale you should be able to upgrade for a significant cost savings (Artist to Professional is currently $99 at JRRShop).
Cake has come a long way since V4. I think you'd be surprised by the capabilities you'd gain even in a lower tier product like home studio - it's pretty awesome and might be all you need.
Best of luck.