A TV tuner card will allow you to watch TV USING your computer. There is no need to have a second device connected to your Dell LCD. Even a hardware-based tuner card ( ATi Theater 550 Pro based ) would cause a hit in gaming performance. You will probably have to run the game in windowed mode in order to access the TV output window. I have this (
http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=327002 ) card. I already had a remote, so I didn't need one AND there is a $20 mail-in-rebate until the 14th. You can choose any 550-based tuner card but this is the best deal. This will also allow you to record TV to your hard drive ( say PVR

). You will still need to connect CATV coax to it.
The second option is to get a cable / satellite receiver that has Composite or S-video outputs and just connect it directly to your LCD. You will have to use the PiP / PBP feature of your LCD to display either the CATV or PC in a window or side-by-side. The drawback here is the limited PiP window size, which is about 1/6th the size of the LCD size. You will need to have audio Y splitters or a small multi-output mixer to route the PC/receiver audio to your speakers.
I have both a tuner card and hook my HD satellite receiver to my Dell 2405FPW using the COMPONENT inputs. I can view TV while surfing the net or have it in a small window while playing some games.
DO NOT get a cheap software tuner card.

A hardware-based card is roughly 2 to 4 times more but will consume no more than 20% of the CPU.