Anyone know anything about Video cards?

I'm looking to upgrade my PC. I was going to let Futureshop handle it but they are giving me a hard time and they have tried to sell me a few things I know I don't need as well as things that are higher priced then I need.

I have a Dell Inspiron 530, with an Intel Core 2 Duo E7300 2.66GHz, I'm pretty sure my power supply is 300W and so I would need a new PSU probably 500W to support a new video card. The video card I was looking at is an ATI Radeon HD4850 512MB GDDR3 http://www.futureshop.ca/catalog/proddetail.asp?sku_id=0665000FS10118965&catid=25617&logon=&langid=EN

I just want to know if my system can support it, what kind of power supply I need and how to install it.

Any help would be great thanks!