I've been there... trying to hook an xbox up to a monitor, i wouldn't go there, especially if the display lacks internal speakers. I'd just get a small LCD in addition, they are pretty cheap nowadays.
the Dells are up to snuff, with AH-IPS in the pricy ones. Might wait until Apple is done refreshing their hardware line, who knows what we'll see there, might even get HDMI input on the TBD refresh. rMBP got HDMI, never thought I'd see that... (HDMI input on the TBD is just as expected, so I wouldn't even hope for it)
Even though i prefer the TBD, that has way more to with the design and it matching the mac than features alone (I'm crazy that way, no... I mean... almost OCD crazy). The Dell just fits better in with what you want... Just do some research on the xbox thing. Alot of people seem to be using the Dells for xbox and other consoles