A fan in the top of WHAT enclosure? The PSU or the case?
If you're talking about the case, then it's not a case with a top mounted power supply. If it's a top mounted power supply style case, then there is not going to be any fan in the way in the top of the case.
For top mounted style cases you face the PSU fan intake down into the case. For bottom mounted PSU style cases you face the fan intake down as well, but it will be facing OUT of the case, towards the floor, not up in to the case.
Besides which, depending on what you are planning to use this WITH and FOR, you should know that these Dell 460w power supplies are pretty much junk.