for 300$ take just one Graphic card...
Nvidia : GTX 770
AMD : HD 7970 GHz Ed, R9 280X
You can choose one of the three cards and pick the one that costs lowest to you..
http://pcpartpicker.com/parts/video-card/#c=137,167,148&sort=a8
For multi-monitor setup, my suggestion will be AMD's R9 280x.. You can run two monitors..
for more than that, two GPUs are recommended.. but for 300 $$ two GPUs will not make sense.. suggest you to game on only one monitor..
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202046&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
Get it quickly.. you will get three games free with them too..
But make sure you have a good PSU also.. something that's about 650 Watts (assuming you will not x-fire) 80+ bronze atleast.. if you want to x-fire or SLI, you will need about 750 watts 80+ bronze standard atleast..
and a good CPU (FX 8350, i5 4670k) so that there's no bottleneck..