With all that downloading you may have picked up somethings you were not aware of. And that could be slowing the PC down.
I would run SpyBot Search and Destroy (freeware), Spywareblaster (freeware) and AdAware (also freeware) to make sure your machine is clean of non-virus malware. Then I would suggest WinASO, (still freeware), that can cleanup bogus registry entries (mostly where the registry is pointing to files that no longer exist). Then I would defrag the hard drive with the Windows defragger.
All this could take some time, probably most of an afternoon, especially the defrag. Spybot will take about 45 minutes to run; it checks for over 200,000 items. Spywareblaster blocks things from happening; it runs rather quick. AdAware has two modes, Smart Scan or Full Scan; Full Scan will take longer. WinASO runs pretty quick, but the freeware version only fixes 10 errors at a time. The real unknown is the defrag.
But I would try this first before spending dollars on an external drive that might not be needed.
I also use CachemanXP to monitor how much free RAM I have. I've got Windows Task Manager set up to run at startup, with an icon down by the date and time. While not exactly perfect, I do have a good idea of how well my box is running.