the PNY RAM is actually a decent price w/o the rebate. not the best, but somewhere in the middle of the pack. the nice thing about giving it a whirl is that if it doesn't work u can just say that upon returning the item and avoid compusa's restocking fee. whatever u go with, make sure to run apple's hardware test on it at least 3x when u plug it in, and be wary of any lockups that u might bump into. i'd say work w/ the computer for about a week b4 sticking the memory in. then, if u have any problems or lockups, u know it's the memory and u can take it back.
be wary of compusa and their rebates. i'm still waiting for mine from 1998 on a hard drive purchase, and personally, i don't think it's gonna come thru.
compusa also just settled a case re: their shoddy rebate offers. i.e.-they don't always follow thru and actually pay ppl the money that they should.
if u want other options, check out dealram
for 256-512MB sticks. as u may have found already newegg
has one of the best priced and best reviewed sticks of 1GB RAM for the Powerbooks or iBooks. Oupost also has a well thought of stick, but i'm not sure offhand which one it is.
re: RAM amount, i think it's more a matter of whether u want to possibly upgrade again in the future. also, if this is the computer for your girlfriend i'd say that a 512MB upgrade w/ a 768MB total is more than enough for a casual user. i throw some pretty intense stuff at mine and it's doesn't wince or hiccup one bit. if she's gonna just be doing casual stuff on it like surfing, email, and typing papers i'd even say u should save some money and just get her 256MB extra for 512MB total.