The keys are interchangeable between 32 bit and 64 bit on Win7 - all you need is a 64 bit install media compatible with your key. Is your key retail or OEM?
It isn't possible to upgrade 32 bit to 64 bit - you have to do a wipe and install. You'll need to obtain a 64 bit install media for your version of Windows, back up your personal data and software, and then do a clean Win7 install.
However, if your system is the one shown in your signature, you'd see some benefit from going to Win7 64 bit given that you have 4GB of system memory and 1GB of video memory.