powershot a40 is basically a bigger version of the ixus v2, although it can use lens adapters like wide angle, telephoto, etc that the v2 can't. i have a v3 and very happy with it, nice and portable (fits into a pocket so well ive taken it clubbing!). a40 lets you use AA batteries (4 i think) which weigh it down but also means you can easily carry spares whereas v2 uses lithium rechargeable, which ive found lasts ages but not as handy as buying AAs when you're on the move.
both will take roughly the same number of photos. to get the best you should put it on top setting, which will get you about 17-19 shots on a 16Mb card. i bought a 128Mb card (£50 now on the high street, even cheaper on the net) so i can fit on loads of snaps and 10-20 mins of movie footage.