So should I upgrade or do a completely new build?

So here is my present Rig:
AMD Phenom II X2 550 @ 3.10Ghz dual core processor
AMD Radeon HD 4350 1 GB GDDR2
2GB corsair Ram @1333Mhz DDR3
Gigabyte GA-MA74GMT-S2 Motherboard(AM3 socket not AM3+)
500Gb Seagate HDD
1 DVD Drive
Cheapass Frontech case and 450W Frontech Powersupply

My priorities are Gaming and video editing but mostly heavy duty gaming
I will only play FIFA 13 and other sports games at 1080p(1920x1080) the rest i will play at 1366x768
I wanna play Battlefield 3 on high
my budget on the Upgrade is 350$ but on a new build is 540$
my preferred website is Flipkart or amazon
Also I know that the PhenoII x4 965 BE is overclockable and it can fit into my motherboard also anyway I am buying a CPU cooler since the stock one has developed some problems and is it worthe to get the Phenom II x4 and overclock it quite a lot and get tthe same performance as an i3 or the FX-6300.. I am willing to get upto 45$ CPU cooler so that i can have the Same performance as the i3 or FX6300 aslo I have set my sights on the HD7850 2GB or the 7870LE so can someone help me here..?