MCE 2005, correct?
Just Install a decent splitter and cable on your TV cable feed. Shutdown, Install the card, attach the cable, boot into Windows. Let it install the driver automatically if your current driver is working fine (might have to reboot). Go into the MCE TV tuner setup and have it configure your tuners automatically so that both tuners are available to MCE.
That should be it. Test it by starting a recording and then changing the channel...you should be able to watch it without stopping the recording.
Theater 550 has onboard cache and hardware encoder ship, so bandwidth shouldn't be an issue at all, especially since they'll be on seperate busses. Drivers shouldn't be an issue if its currently working fine since you'll be using the same drivers.
Edit: Provided your Dell has a spare PCI slot for the card
