When you have the card installed, make sure you plug the monitor into the card's graphics port and not the integrated graphics port.
Is the monitor on the proper input (VGA, DVI, HDMI, etc)?
Did you hook up the 6-pin PCIe power cable to the graphics card?
Also - when I looked up your PC on google, it said 250w PSU, did you upgrade the PSU to a 500w? http://www.powerspec.com/systems/system_specs.phtml?selection=B634