Help! Getting poor performance with Starcraft 2

First of all here are is my system:
Windows 7 32-bit Ultimate
AMD Phenom II x4 965 (C3) @ 3.4GHz
OCZ Gold 2x2GB 800MHz Unganged
Gigabyte MA78GM-US2H Rev. 1.0
ASUS GTX460 DirectCU 1GB @ Stock
Seagate ST3320820AS 320GB / WD Caviar Green 2TB
Corsair TX650

Okay, so here's my issue.
Whenever I play Starcraft 2, I get annoying little stutters when I try to build something / move a group of units.
Also, the framerate dips when it does that, it goes from 60 to 25 and back up to 60, then back down to 25, and so on.
In huge battles the fps just dips to about 28, and it stays in that range(~5~fps) which is very slow and stuttery for proper play.
Also, when the stuttering occurs the hard drive makes some noise, I think the game is trying to load textures from the HDD?
I do not know what could be causing this, and I'm really frustrated.

I tried:
I updated all the drivers I could think of and find.
Every single in-game option did not help, and fiddling with NVIDIA control panel didn't help either.
Messing with Superfetch/Pagefile didn't help.
Tried that variables.txt with framerate cap thing, didn't work either.
Checked all system temperatures, they were all fine (e.g. CPU stays under 50C at load).
Most other common fixes.

So what could be causing this really?
Is it because my HDD is way too slow and I just need to grab a newer and faster hard drive like the Samsung Spinpoint F3/F4?
I would really appreciate some insight
Thanks