upgrading my Dell Optiplex 980

I own a Dell Optiplex 980(minitower) with i7-870,8gb RAM,3 500gb Hard drives and a dell standard 305w PSU

I need advise on upgrading Graphic Cards for it,

for now 1st.  PNY GeForce GT440 1gb GDDR5 with a requirement of 300w PSU (with a minimum 12V current rating of 22A) not sure i have that!!!! will it work?

2nd. Radeon HD5670 with a 512mb or 1gb with a minimum requirement of 400w!! which really sucks!!! will it work?

also is there a more powerful PSU that will work on Optiplex 980?

im a casual gamer, WarCraft3 for DOTA, Modern Warfare 1 and 2 thats it. so i shouldnt need a powerful gaming cards, but i do watch HD movies so it must support good video rendering. preferably with a HDMI(with audio)

I would really appreciate you help.