A new lag?

So, I've got myself a 2011 Macbook Pro. It came with OSX Lion, probably because it was CTO with a 750gb harddrive instead of the first one they choose for you.
Well, that harddrive failed about a month ago after I got in a bit of a wreck and my case got dented, so I replaced it with a 1tb harddrive from WesternDigital. It's a WD10JPVT. I've had no problems with it, and I've hardly noticed any slow down at all. I've been playing a lot of SWKOTOR - the first one - just because.
It's been great.

Well, now I'm having trouble running things again. It starts up fine, just slowly. I can execute programs and such, but there so much lag! Plus, when I start doing things that involve typing - i.e. Word 2011, Facebook, Terminal, this thread - iTunes music stops playing, as though I'm pausing then playing it very quickly. Also, scrolling is pretty bad.

A buddy of mine recommended Memtest OSX. I just bought and ran it, but everything exited with "ok." I dunno what to do now. My HDD is like a month old. I installed it myself, and everything went off without a hitch. The S.M.A.R.T. status is still Verified.

tl;dr My Macbook Pro has started lagging lately for no apparent reason. I think it's the RAM, but Memtest OSX didn't seem to find any problems.

ALL suggestions are welcome. I depend on this thing for school.

Please read the "Beachballs" link in my signature...it provides some possible solutions or reasons for "slowness" issues.

- Nick

Swap an paging were both at 0, but now it won't even boot up. I turn it off before I take it anywhere, including just from one class to another. It works just fine booting in my external recovery HD, but I can't get it to boot from a Ubuntu 11.04 LiveCD I've had lying around.
Any more suggestions? Anybody?