i guess it really depends on the design of your cabinet. I would think that it would probably be easier to build a new, smaller cabinet than try to modify an existing cabinet. what is your cabinet like now? CRT or LCD monitor? they do take up alot of space, but they are so awesome, it is worth it. Personally, I say if you want to make it smaller, then do it. But if you are only doing it because your girlfriend says so, then dont. if you give her that kind of control now, you don't stand a chance if you get married someday. i got my first cabinet a week before i proposed (brought it home when she wasn't there, then told her about it), othewise i would have had a much more difficult time getting one. maybe not though, because i did just manage to talk her into letting my bring a gigantic showcase cabinet home later this week...
to get back on subject, look at some of the examples here, there are tons of examples of bar tops and slim stand up cabinets that dont take alot of room