I had 1.8 20" and 1.9 iSight 17" G5 iMacs and compared to the dual 1.25 MDD I had, the 1.8 was just in front but the the 1.9 was way ahead. The G4 was better for the graphics work I was doing because the software used both processors but for general duties the G5s win.
Also consider for Leopard, a Core Image graphics card would be an extra on the G4, whereas the G5 are compatible already.
Don't forget there's 4 variants of 1.6/1.8 G5 iMacs - the 1.8 ALS is a significant leap from the first 1.6
Certainly in the UK, some early Intel iMacs can be had cheaper than G5s