I'm fairly certain that a Windows 8 key will work for both 32 bit and 64 bit versions. The only catch is that you need to do a complete reinstall, you can't just upgrade.
Your best bet in my opinion, like jebise said, would probably be to upgrade to a 750ti or something of that sort and add some additional RAM. Any bigger upgrades would probably require a different cpu/mobo as to not be bottlenecked by the CPU.
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