I think it really depends on the experience you are looking for.
I have a P4 3ghz with 2GB RAM and a nVidia 6200 AGP card. It plays COD4 fairly well in low resolutions (800x600) with a slight lag now and again.
Branded models will use good parts but likely to be generic. The only plus is that the systems are tested and are covered by the brand's warranty. For example Dell will provide you with up to three years on site service for the WHOLE system. Not just one part. So if the system starts playing up, you do not have to worry about it...
Self build gives you the ultimate control but could end up costing you more, especially if you go for high end components.
Personally, now that I have built one system, my next system will probably be a Dell which will have enough POW to play games but not to a games freak level. Just enough to get the kids of my back....... and then I do not have to worry about - "Dad, the sound has gone" or "Dad, I am getting a BSOD...." :D
I like their XPS430. I think it offers great value for money..... unfortunately for us in the UK, high end components cost an arm and a leg, even on line....
So something a long the lines of the above system - Intel Quad Core CPU (Q8300), 4GB RAM and ATI Radeon 4850.