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Biggest limitation will be the power supply and the available cooling. If you plan to add a videocard you'll find the power supply is under powered for most of the cards you might want to add and/or you don't have enough extra connectors to connect to the video card. Also Dell uses a lot of proprietary fans, heatsinks and shrouds which may cause you to run into overheating issues if you upgrade the CPU beyond a certain level.
On the small form factor models you might find that you can't put a faster CPU on them due to thermal issues even though the chipset and hardware support said chips. Also the max memory in the small form factor models will be gimped as opposed to the same model in desktop flavor. say the desktop version of the system supports up to 16GB RAM max the SFF may only support 4 or 8..
When one talks about media center PCs, the assumption is that one means a mini-tower in an office setting and a CE-friendly desktop design in the living room. Strictly speaking, these aren't really small form factors, although the sleek styling from case manufacturers like Antec and SilverStone sure make you think twice. Fortunately, microATX does allow for some size shavings beyond these dimensions.