Hi, it's my first post here

Here's the situation. I switched from XP to Ubuntu and I'm really happy about it but my experience (which was already just about "correct" on XP) in the only game I play (StarCraft 2) is now really critic in 4v4 format.

Here is the heart of my PC :
Motherboard = ASUS P5N-E SLI NVIDIA NFORCE 650I SLI C55
CPU = INTEL E4500 CORE 2 DUO BOX
RAM = CRUCIAL BALLISTIX TRACER 1GO DDR2 PC6400 (X2)
Graphic card = ASUS VGA EN8600GT SILENT/HTDP/256M

Do you think a <100 graphic card could give me the little boost I need ? I'm not entirely sure about it as my CPU is also in concern to my mind. Yet, it is out of the question to change CPU because the motherbord is already obsolete (but for now, I have no use of USB 3.0, SATA 3.0, PCI E 3.0, etc.). Moreover, you can anderstand I don't want to renew MB+CPU+RAM just for a game. So, the problem is : Is a new graphic card would give me more FPS even with an already very busy CPU ? I heard the GPU load the CPU.. so I wonder if it would really be a good thing to put a 2-3X more powerful graphic card on a CPU already crowded in SC2. And just as a note, I heard too that SC2 was a very CPU focused game. It is particularly true for me because I use low settings (easier to anderstand what's going on => pro-gamer style ).

If the answer is yes, then I'd like to know about a fanless graphic card well supported on Ubuntu that could do the trick !

I already had a look and wonder about those :
ASUS ENGT440 DC SL/DI/1GD3
HIS H667PS1G iSilence 4 - Radeon HD6670 - 1024 Mo

Thank you for your support !