Building ~500$ need advice

Approximate Purchase Date: a couple of months

Budget Range: 400-600

System Usage from Most to Least Important: Gaming,Surfing the internet,watching movies

Parts Not Required: Speakers, i have windows xp not sure if i have to upgrade. i also got case, mobo, PSU. i just need CPU and Graphics card im assuming

Preferred Website(s) for Parts: canadacomputers.com

Country: Canada

Parts Preferences: AMD CPU and NVIDIA GPU

Overclocking: if requierd

SLI or Crossfire: ??

Monitor Resolution: no clue all i know is its a 24 IN gateway

Additional Comments: im FIXING/upgrading a computer, the GPU and CPU are 4 years outdated. it turns off when i open things too fast or move the mouse too fast. wierd. also i would like it to run BF3 of ultra