8gb or 16gb?

I'm going to buy a upgade for my memory earlier, but I remember that I don't know any of this stuff if I'll buy x2 4gb DDR3 or x2 8gb DDR3? because I don't know what to buy and what type of brand of memory, but I'm going to upgrade my Palit GeForce 210 1GDDR3 to Asus GeForce GTX 660 Ti DirectCU II Top Edition 2GGDR5, I need a memory that is for gaming and can run applications at the same time without lagging/hanging your computer, I'm using Windows 7 Ultimate 64bit and have a memory of 4gb DDR3.

here's my specs ( links available ):

Motherboard: Gigabyte H61M-DS2 rev 2.0
http://www.gigabyte.com/products/p [...] id=4073#ov

Processor: Intel i5-3570 6M 3.40Ghz
http://ark.intel.com/products/6570 [...] e-3_40-GHz

( Graphics Card: Palit GeForce 210 1GDDR3
http://www.geforce.com/hardware/de [...] eforce-210
http://www.palit.biz/palit/vgapro.php?id=1410

upgrade to Asus GeForce GTX 660 Ti DirectCU II Top Edition
http://www.asus.com/Graphics_Cards/GTX660_TIDC2T2GD5/ )

Memory: 4gb DDR3
Hard Drive: 500gb 7200rpm ( partitioned 200gb/300gb )
Operating System: Windows 7 Ultimate 64bit
PSU: 500W or 600W I haven't check it out lately