Since you're buying a prebuilt system, just stick with what they give you. If you were building it yourself, I would say get 64bit, but the benefits are negligible. The main difference is amount of ram the os can support, the 32bit caps out at 4, and only recognizes a little over 3. Since your laptop has two, its more than enough for anything you need right now, even if vista is ram hungry, four is probably overkill. So, to sum everything up, its a waste of your time to upgrade to 64bit if it comes with 32bit already installed.
EDIT: stay away from xp 64-bit, it has more compatibility problems than vista. Lots of games just don't even start in xp64, its a huge waste of your time and effort. On a different note, I have vista installed on one machine right now, and its perfectly stable and smooth. You can look into if you want, but paying more for an older os just doesn't make sense, vista will be the standard in time, and as they patch it and give it more time, I'm sure you'll be happy.
)