The Optiplex uses Dell proprietary motherboards and connectors. Other than installing a more capable Optiplex motherboard, you really can't upgrade the system very much. You can max out RAM and add a more capable video card (assuming your PSU is up to the task).
Not really, in fact, you would only be able to connect another Dell (proprietary) heatsink/fan assembly to the Optiplex motherboard. You might be able to put one of the XPS heatsinks on the Opty, but your fans and shrouds may not line up properly.