Need help upgrading (CPU bound) PC for better framerate.

[Fleegsta]

I'll try and keep this short and concise:

Basically, I want the cheapest set of upgrades that would allow me to play the game on Ultra everything with a 30 (preferably 40) fps minimum (meaning in the largest of battles) on my 1080p monitor. I'm willing to overclock and whatever else if ideal. I have no limited budget - I have quite a bit of extra cash saved up, I'm not looking for the best build to satisfy a specified price, but rather the opposite; the cheapest build to satisfy a specified performance. Keep in mind I'm still fairly newbie at PC building, so please be specific, concise, and detailed (especially on terminology, easily confusable part names, risks and requirements, etc).

My current PC build has been able to run every other modern game
so far on ultra with a 30 fps minimum (excluding occasional spikes from loading stuff) no problem. It was custom built by me a bit over 2 years ago and as proved quite the beast at getting the job done.. up until this one game.

In PS2 (running gamebooster, unparked cores, CPU efficient game settings (except draw distance)) my fps varies dramatically (and seemingly randomly) from 20-65 when alone (usually only drops below 35 when certain events happen). In large battles, it varies from 1-20, making it impossible to play competitively like I naturally like to do.

My Current Specs:
Processor: Pentium(R) Dual-Core CPU E5300 @ 2.60GHz (2 CPUs), ~2.6GHz
RAM: 4 Gig (EDIT: DDR2 I'm pretty sure, as the mobo slots are DDR2)
Video Card: AMD Radeon HD 6870
Video Memory: 2388MB
Power Supply: 700W
Operating System: Windows 7 64-bit
EDIT: Motherboard: ASRock G965M-S (http://www.asrock.com/mb/Intel/G965M-S/)
(I have no idea what fan I have.. Forget how I'd check that, can't seem to find the boxes and apparently not listed in dxdiag).

I assume the primary problem is my CPU.. I highly doubt my video card would need replacing as it's fairly top tier and was a recent upgrade when StarCraft 2 came out to run it on ultra (which it does flawlessly). Plus GPU's are expensive and I'd hate having to upgrade this pretty damn good one. Not to mention I just realized it's the recommended GPU in the game's recommended settings.

So again, what would be the cheapest set of upgrades to get me a 30 (preferably 40) fps minimum in any and all scenarios (including large scale battles) on ULTRA everything.

And, if it matters, I don't ever turn vsync on in any game. I don't even understand why it exists.. When it's not causing input lag it seems to do absolutely nothing anyway. Motionblur would be nice but I might end up turning it off for competetive reasons anyway (as I often do). EDIT: Oh yeah, and despite my competetiveness, I want to be able to keep shadows on.. It just makes the game look so much better it's worth it for me.