OK, so I've done all of what mostapha suggested (except defrag due to the fact that it would take DAYS to do), starting from the first item on the list to the last.
Repairing permissions really seemed to help and almost got me back to 100%. But I'm STILL getting really annoying hangs in various programs, ESPECIALLY in web browsers. I've tried Firefox and Chrome and Safari and they all seem to do it Basically, click on a link, program (not computer) freezes, get a pinwheel, and then I get function back after the page is fully loaded or action is completed. It even does this on startup when loading my home page (www.surfline.com). The more programs I have open, the worse it is.
Fortunately, Traktor seems to be pretty bulletproof in terms of audio output. I might get an unresponsive screen, but whatever action I take is executed on immediately and I've only had one audio dropout with iTunes open, a 250gb file transfer running in the background, and 50gb of torrents going, too. Wanted to stress test it.
So, at this point, I'm REALLY close to 100%, and would LOVE to get the browser issue resolved (sonofabitch, if it didn't just take 20 seconds for another tab of the forums to load, all while pinwheeling!). Any suggestions, anyone? Any common programs or processes that might be running in the background that I should check for?
EDIT: Or is this just something I should expect with 450GB of data on my drive?
Thanks!