I have the mini-tower and it may support 125W CPUs. From http://en.community.dell.com/suppo
The Optiplex comes in three models.
Mini-tower (Model #DCSM),
Desktop (Model #DCNE) and
Small-form Factor (Model #DCCY).
Each has a different size power supply/Motherboard and only the mini- tower will support a 125w processor.