See, I use a mere 500MHz G4 for nearly all of my computer work...single processor, the computer used SDRAM and a 100MHz system bus and has plenty of RAM.
I can use it for all my Photoshop purposes, all of my audio editing with Sound Studio, all of my basic workings (email, using a browser, managing files), most of my programming, most of my iPhoto functionality (using iPhoto 2), and some of my Quicktime Pro video exports. So a dual 1.42GHz G4 with DDR-RAM would kick my computer's ass (unless I added
blue lighting underneath my Cube, like some respected dude did). Thus, if you plan to use your G4 purposes, then you can probably get a sense of how well it will perform for certain operations.
One comparison you can do with a Mac and PC is: Applications that rely a lot on the L1 cache will run faster on a Mac, given frequency to frequency comparisons, because the P4 uses such small L1 cache sizes (earlier models used only 4KB, I think), while the Mirrored Door Powermacs use 64KB.