Originally posted by: Kiwi
The answer is like ever so many computer-oriented things: "It depends".
If it's an SFF box, no. If it's a full-size enclosure, an ATX format MB certainly should fit.
THERE IS A 2nd PROVISO. Most brand name sellers have proprietary connections from psu to MB; you will normally have to get a new psu also. Dell does periodically put on promotions of "Gamer Special Systems" that contain all standard ATX parts, and are priced higher than their usual as a result. These are not always available, and just because one Dell box or another may have been advertised as suitable for gaming doesn't mean it was one of those all-standard-parts specials.