Agreed with two prior replies. For what you're doing, any mac should be sufficient.
I doubt there will be any significant upgrade to the imacs, since they were recently updated. The only reason to wait til WWDC is to see if snow leopard, OSX 10.6, is released then. Otherwise, you'll have to pay $130, maybe less, to upgrade to the new OS once it's released.
If you're a student, you can wait for the back to school special in the summer, where they throw in a free ipod with the purchase of many macs. Or buy a refurbished unit off of the apple online store now to save a couple of bucks.
If you've got a pc that can minimally meet your demands, you may nurse a few more months out of it to see what Apple does. If you don't, you may as well have one now.