$~350 PSU and GPU Upgrade Help

Approximate Purchase Date: e.g.: December 25th 2012

Budget Range: $300-$500

System Usage from Most to Least Important: Gaming, Video Editing, Recording, Videos, Surfing Internet

Are you buying a monitor: No

Parts to Upgrade: (e.g.: CPU, mobo, RAM) PSU/CPU/RAM

Do you need to buy OS: No

Preferred Website(s) for Parts: Newegg.ca / NCIX.ca / MemoryExpress.com / TigerDirect.ca

Location: Calgary, AB, Canada.

Parts Preferences: No specific brands. Whatever works.

Overclocking: No

SLI or Crossfire: No

Your Monitor Resolution: 1920x1080

Additional Comments: Currently have a Dell Studio XPS 8100, want to upgrade a replace parts. Currently have 6gb of ram and for what I am doing, should I upgrade to 8gb or even 16gb? Also, if its worth it, should I just build a new computer and gain that experience? If so, budget will change to ~$750-900

And Most Importantly, Why Are You Upgrading: Mainly for gaming, only getting around 22-29fps in Guild Wars 2 on lowest setting.

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Currently, I have a Dell Studio XPS 8100, 6gb of Ram (DDR3 1066-1333Mhz Available), Radeon HD 5450.
I would like to get the best psu and gcard for around ~$300-500, unless reccomending I build a brand new computer.

I am 17, so this would be the first build for me, but I would love to gain that experience, I will be going into Computer Science in University.

Any help is appreciated! Thanks for your time