buy, Buy, BUy, BUY!
Unless you don't need the PB now, you should buy now-there's no way Apple will upgrade now...they already said it would be 'quite some time' until they get a G5 into a PB...of course if you can wait until the convention, it'll put your mind at ease. If you do wait, and there is no boost, don't get fooled into waiting until the 'next convention'...it's simply going to be a LONG time...
BTW, have you checked out Macrumors' advice?
<https://buyersguide.macrumors.com//>
BUY!