What should I upgrade on my Computer to be able to run Starcraft 2 on Medium settings with absolutely no lag even in a 200/200 fight.

AMD Phenom(tm) II X3 720 Processor 2.80 GHz

4.00 GB RAM

ATI Radeon HD 5770 Graphic Card

Please ask for more information if you need it! Also, can you recommend something from a website where Paypal can be used and that ships to Canada! Something like www.newegg.ca