Graphics card help for gateway dx4300-11

I wanted to get a new graphics card for my computer that i just got a few days ago. The one in here is pretty crappy.. lol. I was looking at getting either Radeon HD 4850 or 4870. But I'm not sure if the computers power supply would be enough for it, nor if since the computers current graphics card is intergrated, if it could take a separate graphics card. Sorry if that sounds like a dumb question, but I'm not very educated with these things. Also, if it makes any difference, the monitor I'm using is the LG W2361V-PF, hooked up through HDMI. Any help is appreciated. Thank you!

Dino