Is this for offline, LAN or online SCII?

If it's online, it can just be your internet connection that is lagging out. (or your network controller)
I have the same GPU and a bit better CPU (HD5870 / I7-2600K) with 16GB of RAM and play the game fine, both offline as multiplayer.

But I do have 120Mbit/30Mbit download/upload on cable, my friend has a much lower-rated internet subscription (20Mbit/5Mbit on ADSL) and he lags like hell with much units in battle with a better and newer PC. (plays fine singleplayer).

Might look at that before investing a lot of money in hardware whilst the problem lays elsewhere.