Can I get away with just upgrading the GPU?



So Im wanting to upgrade my 3 year old Alienware. And as the title suggests I am unsure If I could get a bit more life out of it by just throwing in a newer GPU or are will I still be noticeably bottlenecked by some other areas. Current specs are

Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz
2x NVIDIA GeForce GTX 260 for a total of 1792MB graphics memory in SLI
6G PC3-12800 (1600 MHz)
Windows 7 64 bit

Anyways I think I could just throw in like a GTX 560 or two and notice a huge difference but I'm in a quandary. My current plan is to upgrade piece by piece as money allows and start with the GPU. Anyways any thoughts and insight would be appreciated. I'm no tech guru by any means but I'm not a complete idiot either, my biggest problem is keeping up with how fast it all changes.
Thanks in advance.


In my opinion, upgrading the cpu would be a waste of time without upgrading the gpu as well, as theyre fairly outdated cards now.