I think you would be disappointed. I would suggest that your 2.5 GHz PowerMac will exhibit about the same performance as an equally clocked iMac. Even with the 2.8 GHz Extreme Edition, you will not see any major delta.
You should also consider what you are doing with your Mac, and how important upgradability is. The G5 is a more flexible machine than an iMac from this perspective.
I would also gently suggest that just because the machine and/or display is out of warranty is not a reason to replace it. If it continues to meet your needs, I would stick with it. I have a dual 2.3 GHz G5, and I am doing just this - I am sticking with it. Someday, when Apple comes out with a quad core 3.0 GHz+ 24" iMac, I may consider a change - at that point, there would be a significant performance jump that would justify it.