Do i really need to replace my whole computer?

Hi, I' hoping that someone can shed light on what I really need to purchase in order to achieve my goal, Which is to have the ability to play some generally newer games well enough to not find my self feeling handicapped by frame rates.

In the Past ive played World of Warcraft, More recently Guild wars 2 and Starcraft 2. Some offline Beautiful games like The Witcher 2 and Skyrim, In all these games as the graphics get better im noticing my System falling behind but never enough that i cant tweak or find a way to bring them back up to 60 Frames per Second. Until Battlefield 3,

My system is:
Windows 8 64bit
Asus m3n78-em
AMD Phenom IIx4 955BE
8gb GSkill DDR2 800 ram
EVGA GTX260 GPU
Corsair 650w PSU

If there more Parts that are relevant i can find out what they are.

I feel like the wall i keep running into is DX11, Some of the games above allow you to select dx9/10-11 in game when your adjusting video settings. And almost consistently when i select DX11 as the option I fall below 10 fps, irrelevant of my resolution or any other settings i lower or raise, and i find that when i avoid using DX11 as an option i can have Medium/High some cases Ultra settings without coming away from 59-60-61 FPS almost reaching 100 FPS without Vertical Sync enabled. So unless I'm Missing something, my system is not new obviously but also not Weak either just outdated.

But Recently I tried to play Battlefield 3 and i found that The ability to Choose DX11 over 10 or 9 was not available.
With the Settings on Medium where i can choose it and high on textures, SSAO enabled and the settings refering to filtering above SSAO turned to 1x or off. I can get 35-40 FPS in campaign but when i go into multiplayer where i want to be i fall into 12-17 fps, Completely destroying my ability to enjoy the game, And im sure it dips lower its just 12-17 when im checking.

I have OverClocked the GTX 260 to 666 clock and 1150 memory to achieve this. Dropping all settings to low or putting them to high does not seem to have any effect on the frame rates lower or higher. Except Shadows Shadows needs to be on low or it brings me below 7 FPS.

Normally i would just buy a new machine and save the time thinking about it, choosing a machine is fun anyways, but i need to buy a Notebook for my Fiance this time around for photography. I cant just go drop 2000 Dollars on a gaming tower.

So, I basicly need to know, If i can buy a new Graphics card and make it last 6 more months till i build a all new one. or will i be swapping out the Motherboard anyways to be accomodating a Graphics Card worth having for Battlefield 3 and all other DX11 Games. My reason for focusing on the GPU is that mine only has support for DX10.

Please help and thanks for reading.