What's the noticeable speed difference between these two Mac computers?
Having a look on everymac.com i see the following Geekbench measurements

MBP pro     32bit=9991/64bit=10823

Imac 2007   32bit=3216/64bit=3595

with this info i would say yes. by a whole lot..

I'm writing this info funnily enough on a 2007 imac 20 inch so i think its a good machine...and it still going strong but it has its limitations that become apparent as soon as you leave OSX 10.6 and go to 10.7 or 10.8...

So if you use your mac for lets say web browsing and light apps then it does its job... its memory limit is 6GB by the way...

But if you want to use it for music/video editing creating etc.. With current versions of suitable applications then i would say that would be a GREAT downgrade...

Also if its any help according to everymac.com:

MBP pro  estimated current price US$1200-US$1350

Imac2007 estimated current price US$750-US$850