Questions about q9550 and upgrade 6870 -660 gtx ti

Hello,
Hey guys,

My system is:
dell monitor: u2410 resolution 1920*1200
Win 7 32 bit
Processor q9550 mildly overclocked to 3.2 ghz
Motherboard p45 neo f
Ram 3 gb ddr2 now but had 4 gb ddr2 before and no performance difference after the degrade
Graphics Card ati 6870 on stock


Ok..So my questions is, will i get significant boost in fps while playing the game on high after upgrading to 660 gtx ti

Right now I get frames of 40-65 on high settings, very rarely going under 40. On air maps frame rate stays between 40-50 and on smaller maps like metro it dwindles between 45-60. My main problem with playing on high is that the frame rate suddenly changes in heavy firefights ie. it goes from 50-40 in one second making me feel like the game is not smooth. I am posting a screen shot just after i died in a firefight when my computer was stressed to almost its maximum.

http://i45.tinypic.com/httst5.png