What type of stuff are you using the PB for? I bought a 1 Gig chip for my PB, but hardly ever use the majority of it. If you not doing anything super high stress, I would say just adding another 512 will do you more than fine.
Although if your planning on keeping the PB for a long time, it might just be wiser to spend the money now, and be done with it.
BTW, I think crucial is having a 5-10% off sale this week. Saw an banner ad here somewhere for it.