New Cheap Graphic Card Compatability

Hi everyone, I bought a Dell Optiplex GX520 the other day which included a Acer AL1721 monitor and a keyboard and mouse for $100 australian,it was used by the way(I reckon that's pretty good for a working computer). It's been upgraded so I have 2 gigs of ram and a wifi card already installed before i bought it. Now the question I wanted to ask was If i could get a graphic card around 50 bucks ,70 max and if the computer would support it? I was looking at a Nvidia Geforce 8800 gts with 320mb of memory for around 50 or so bucks. So I was just wondering if my computer could handle it because it only has a 220 watt psu which i don't think can handle it but i don't know that much about psu's i'm pretty clueless when it comes to them but the other question was if i do get this card will i be able to fit it in the case. The dell is just the desktop computer so the middle sized one out of the series and if i do get this will it help gaming performance at a 1280x1024 resolution with only 2 gigs of ram and a pentium 4 proccessor? By the way i'm only playing rts games with hopefully no lag because the intel gma thing just doesn't cut it.