music creator needing upgrade help, thanks!

I am currently running pro tools on an old powerbook g4 laptop. To finish my latest project I need more power. Should I buy the new imac or try to find an old used mac tower. All I can find so far have been powermac g5's or similar but is that better than a new imac? Given the fact that they are 5 years old or more. I wish I had the cash for a new MacPro but records don't sell like they used to. Any and all input will be greatly appreciated.