Hello yui and welcome to Seven Forums.
First thing I'd do is install the Windows 7 Upgrade Advisor on your 4520 and see what it says. You're biggest issue will probably be with the graphics. Successfully running all the eye-candy offered by 7 may be a problem. Download Details - Microsoft Download Center - Windows 7 Upgrade Advisor
This Microsoft article offers some more general information, including "Which version of Windows 7 should I install: the 32-bit version or the 64-bit version?" 32-bit and 64-bit Windows: frequently asked questions
Most 32-bit programs run just fine on a 64-bit machine. You may have some issues with antivirus programs (not very common any more as most major players have come over to the 64-bit side) and hardware drivers
. Here's where it could be sticky as Microsoft can't force hardware manufacturers to upgrade their drivers. But you can usually find an older Vista driver
that will work in compatibility mode. Hope this helps.
EDIT: Sorry. Just saw your added comment about using Windows 7 with 2GB RAM. Obviously, you know7 will work. But going to 64-bit really won't help and might hurt because of drivers. If you were using or planning on using 4GB or more then 64-bit could helps you squeeze out every GB.