You are stuck with the Dell heatsink/fan system. Dell uses proprietary heatsinks and motherboards, so you are stuck with what they offer.
You should also verify that your PSU meets the power needs of the 460 before you buy it. Dell PSUs are nototiously underpowered and aren't really designed to handle the load of gaming GPUs.
I just stuck my head into an Optiplex 760 (very similar to the 755) and I think you are out of luck with a large GPU like the 460. You need to go with something smaller. What are you using now?