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#11
Why would you pay for 64 bit version? You already own a $100+ value license which can be used with either 32- or 64- bit for the life of that machine. You only need to find a 64 bit installer to reinstall.
To know whether it is RAM (likely) or CPU which is needing upgrade most, watch the charts in the Task Manager's performance tab during multi-tasking to see which spikes most.
Besides running a lightweight effective AV like Microsoft Security Essentials with the Win7 firewall, a few things you can do to save resources are to run a clean boot by unchecking everything in msconfig>Startup and >Services (After hiding All MS Services) except AV, then checking back regularly to make sure nothing sneaks back in; edit Visual effects of animation, sliding, fading and dragging intact at Computer>Properties>Advanced Settings>Performance; run the CCleaner Disk and Registry tabs, boot-time defrag using Puran freeware.