i'd say there's a good chance they'd have it in store. and for the 1x1GB chip, i'd guess that they'd just swap that over for you.
but why not just call the store and ask?
as for your PB - is that the 1.25GHz model? i've got the same, and i'm also selling. i've been looking on eBay Australia to figure out the going prices (i havne't been keeping an eye on prices the last couple of years), and i think i'll be lucky to get AU$2000 - or around that amount, for it. although i've got 1.5GB RAM and will be selling a couple of other peripherals with it.
all i can suggest is that you look on eBay and other classifieds sites to work out a price.