I have a base model 2007 2.0 GHz iMac (128MB 2400XT) and my brother has a 2008 2.66 GHz model (256MB 2600 Pro) and the difference is pretty marginal. Both are on El Capitan, both have 4GB of RAM, 2007 has a 120GB SSD and his '08 has a 512GB model.
The biggest difference is UI performance and frame-rate with the better GPU he has under Yosemite and up. Otherwise, websites load about the same, apps open just as quick, etc. If you don't game or edit videos, it won't be very significant. Just feels a bit better to use. A real upgrade would be moving to a Core i5+ iMac with a better GPU too rather than between C2D models that all feel pretty much the same from 2007-2009.