i don't know where you got this from... G5 PBs aren't expected any time at all...
anyway, what difference does it make from usage point of view if G5 PB comes out? were you (or your sister) planning on rushing out to get G5 optimized software all of the sudden if G5 PB came out? if you think semi-rationally, the only reason an updated computer would bother you is because you lose the ever-so-elusive "owning the top of the line" computer feeling. regardless of when you buy a computer, you lose that in about half a year. so the wise thing is, in general**, get the computer when you want to start using it. then once you start using one, don't look back - what difference does it make? computers are getting faster and faster all the time anyway.
** i said in general because with macs, it's probably not a good idea to buy a computer right before MWSF, for example. but since there's no big apple conference until the summer, it's anyone's guess when the updates will next occur...