Go to www.crucial.com and look up your system. It will tell you how much, and what kind of memory your laptop can handle.
I'd say neither at this point. Windows 7 trumps both of them, but I have to agree with the others. Unless you plan to upgrade your memory again, there's no need to consider it.
Even if you decide to stick with 32-bit, you will get a noticeable performance boost by increasing your memory to the maximum that your laptop can take. If you are increasing your RAM, I would replace all sticks at the same time so that you are using memory that is all of identical specifications.
You should be OK with Windows 7 64-bit as its support for hardware is vastly improved compared with Vista (although you should check your laptop manufacturer's website for 64-bit support). As far as 64-bit XP is concerned, I would steer clear of it as the 64-bit support on that OS is somewhat flakey.
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