If you can wait, and can tolerate Windows for awhile, you can wait. But if you want something now, a 1.25 with lots of RAM (and harvested parts from your PC if you want to keep your drives and all) will be fine.
If speed is no big deal, buy now and you'll be happy for quite some time. Keep your Athlon for awhile to get comfortable. When you can afford it later, buy something faster. Or buy the 1.6 G5 if you want the latest tech. It is quieter, but the new 1.25 G4 isn't THAT loud. And it isn't that slow either.
Buy from someone other than Apple (unless you want to configure it, have an education discount, or want to pick it up at an AppleStore), and you might get some freebies and deals.
MacSolutions has some good deals, $39 for 512 MB RAM, cheap AppleCare, cheap shipping, sometimes no tax.
PowerMax has some good deals, too, especially on used and refurbs. Plus it's in Oregon, so no tax.
Just think of it this way though, the longer you have to wait for a new machine, the longer you're without that new Mac. If you buy now, you'll be happy now.
To answer your original question, the 1.25 GHz G4 CPU is much faster than a 1.4 GHz Athlon CPU.