Supahos has a point. The Optiplexes have a 95W CPU limit. Upgrading to the Dell 4 core heatsink is usually sufficient.
The original Dell fan will have an Amperage on the label. Dell tends to use higher powered industrial fans. They use different manufacturers and they don't all use the same rating system. Delta and Nydec tend to use peak startup amps, Foxconn and AVC tend to use continuos running Amps. MTB and Sunon I'm not sure. A 120x38mm 1.2A. Delta and a .9A. Foxconn move the same amount of air. Aftermarket 120x25mmm fans tend to be in the .2A.-.3A. range.
It will probably be OK. But it also won't make much difference.