Honestly, if you are looking at buying a PC for gaming I highly recommend you build one yourself. The main advantage is that you can upgrade your PC for future demand without having to replace the entire thing (pre-built PCs have proprietary issues). It may take a week or two to become educated on the subject, but for $1200 it would be well worth it.
That being said, if you are still set on buying a pre-built dell, I recommend you customize the Alienware Aurora:http://configure.us.dell.com/dellstore/config.aspx?c=us&cs=19&l=en&oc=DPCWVQ1&s=dhs
Go with a 5770 for the video card and 4GB DDR3 1333Mhz RAM for a total of $1149.00. As for the CPU, if you want the quad it's going to cost you $200 more but the i3 530 is still quite capable.