One thing to consider is that a lot of pre-built systems use proprietary power connectors to power the motherboard, these days.  You should check what connectors are plugged into your motherboard (normally a 20/24pin ATX connector and a 4 or 8pin ATX/EPS power connector.  However, some Dell/HP/Lenovo motherboards may use an 8pin (or close to 8pins) connector to power the motherboard (and no 20/24pin), which is different than the 8pin connector used for graphics cards or CPU connectors.
 
These proprietary connectors will make connecting a new power supply a hassle, so you should first check to see if you'll run into this problem.  There are some adapters that can be used to make a regular power supply work with these systems, but it's not ideal.