Buy now or after the next release?

Aside from the obvious thing of faster processors (and it's questionable how much faster 8 cores are vs the 4 we have now in anything except for the most demanding number crunching/image processing applications) I can't really see any major changes to the hardware in the next rev. Maybe Blu-ray and going back to standard RAM but who really knows. What I can tell you is that I have yet to see this thing max out all of it processors. Photoshop feels as fast as a dual 2.0 G5 (which is a lot faster than your G4) and the machine seems well built and pretty much bug free.

If you are going to play the waiting game then I suggest you buy immediately after a release. That way, whatever you buy will be current for 6 months or so and won't be replaced/upgraded soon after you've bought.

As for Blu-ray, I personally wouldn't think this will be anything but a CTO option for quite a while assuming they do become available any time soon. The cost of the drive are just too much for Apple to make them standard equipment. So as a CTO it'll be cheaper to do it as a DIY job anyway.

There will always be something else coming around the corner. It always comes down to how any corners are you prepared to go around. My advise is to buy when you finances asy its right and when you are happy with the system that available in front of you at that time. While I can't see it happening under Intel, the Mac Pro was released 12 months after the Quad-G5 so you can sometimes be in for quite a wait if you're waiting for "the next one".