HERE <ADMIN NOTE: Broken link has been removed from this post by Dell> is the Dell manual.  The drawing for the motherboard shows a heatsink that sounds different than what is installed.

Looking at that image, the pictured cooler is definitely my heatsink - it has the six protrusions on the top.  The image shows what looks like a possible fan between the heat sink and the back of the case, and it is described as a "heat sink and blower assembly" in the legend.  I'm going to guess that it's probably unsafe to use the heatsink alone, even given its size - does anyone have a part number for the blower?  It's not listed when I check under my computer's service number, there's only a line item for the heat sink.

There is a power connection shown on the motherboard for a fan so there is supposed to be one.  But that is all I find in the Dell documentation.

Have you tried THIS <ADMIN NOTE: Broken link has been removed / replaced from this post by Dell>

for parts and upgrades?

The parts site was no help, but I just found what I need through eBay - it's a T4307 fan/shroud assembly.  Thanks for the help!

The NEO Skydive  Case on the left has the fan on the case and should have a shroud over it.

The Mr Smith case on the Right uses a fan shrould on the heatsink and draws air thru the front.

The mr smith case does not have a fan attached to the rear of the case like the Neo (Skydive chassis which is the same kind as the GX240 tower)

Might Be a T4307  with holes for the Heatpipes.    Depends if the Tower is a (Neo) Skydive Chassis or (Mr Smith Tower Chassis) which uses GX003 / HN985 Heatsink assembly and fan.

http://www.impactcomputers.com/t4307-rb.html

http://www.impactcomputers.com/xc020.html

http://www.impactcomputers.com/t4307.html

Notes: T4307Includes :N4664(Shroud ) , F3888(CASE ) , P2780(fan ) . CPU Cooling Fan (2 ) & Shroud Assembly (Includes Fan , Shrouds & Rubber Groumets )

Dimension 8400, OptiPlex GX280, OptiPlex GX280 Desktop, OptiPlex GX280 Small Form Factor,
Precision WorkStation 370, Precision WorkStation 370 Desktop, Precision WorkStation 370 Mini-Tower

http://www.impactcomputers.com/xc020.html

Notes: XC020 Fan and Shroud Assy , Large , Composed by :N4664(green enclosure ) + F3888(black enclosure ) + G5883(EFC0912BF) fan .

OptiPlex GX280

Thanks for the detailed info - I have the NEO Skydive Case pictured, so it looks like I was right about the T4307.  I do need the whole T4307 package, as my computer came with only the heat sink in place.