Upgrade PSU or SLI

Hey guys, this is my 2nd time posting here.

I'm basically just getting ready for the release of Modern Warfare 2. I'm trying to get the best FPS out of the game and I'm about to do some upgrading w/ somewhat limited money, about $300.



Here are my stats:

AMD Phenom(tm) 9850 Quad-Core Processor (4 CPUs), ~2.5GHz
4096MB RAM
NVIDIA GeForce GTX 260
Windows 7 (I have it preordered)
Asus M3A78-EM
550PSU
GIGABYTE GA-MA790X-UD4P AM3/AM2+/AM2 AMD 790X ATX AMD Motherboard - Retail
(http://www.newegg.com/Product/Product.aspx?Item=N82E168...)



Do you guys think it would serve me better to SLI another GTX 260 Superclocked.
(http://www.newegg.com/Product/Product.aspx?Item=N82E168...)

Or would it be better for me to go and get a Phenom II X4
(http://www.newegg.com/Product/Product.aspx?Item=N82E168...)

P.S. I recenty just OC my phenom x4 to 3.0GHz

P.S.S. And lastly if I went SLI would this be enough power? or do I not need to upgrade?
(http://www.newegg.com/Product/Product.aspx?Item=N82E168...)

I appreciate all responses, thanks guys.

-The3rdNipple