Well, since this is OEM pre-installed, you have the following option:
1) Image your system using a program like Macrium Reflect, also make recovery disks using whatever inbuilt app Asus provides. Backup user data.
Now, call Asus and request them to provide a 64 bit disk. If that works, well and good, otherwise borrow a 64 bit disk from a friend and clean install Windows 7 64 bit. NO, you cant upgrade. After that you'll need to call MS and do a phone activation.
2) Since you have already paid for Windows 7, it doesnt make sense to pay once more.
3) How much memory does the system have, 64 bit basically enables use of >4gb ram, if you have less, stick to 32 bit.