Thanks for your suggestions; this one is solved. The root cause of my trouble was a mixture of my own arrogance and lack of thoroughness.
Long story short, the card is quiet and seems to work flawlessly. It eats The Sims 3 for breakfast and then asks for seconds.
Long story:
It started with "okay, sweetie, I'll build it myself and we can save fifty bucks, and you'll be Simming within a hour or two". Much later that night I was at my wit's end and posted my original post on this thread.
I figured it out the next day. I had had the feeling that something wasn't adding up... the somewhat-generic paperwork that came with the card instructed me to connect a power connection to the card, and this I had not done, reasoning that since I could not see any connector on the card (while installed on the motherboard) that this particular card drew all its power off the motherboard and had no second connection and hence the generic instruction did not apply in this case. Wrong! The supply end is a 6-socket connection on the mess of power cables coming off the power supply on the case. The receiving end is a 6-pin connection which is recessed within one of the vent ports on the ATI card's plastic housing. I had mistakenly reasoned that the connection did not exist since it was not visible while the card was installed on the motherboard. After all, I thought, any power socket would surely be exposed and easy to find. Turns out that when this connection is not made, and only the PCI-Express connection is made to the motherboard, then Catalyst Control Center refuses to install properly.
I *could have* removed the card and gave it a closer look first, but no, my mind refused to acknowlege that as an option and so instead I phoned the retailer and we agreed to get the workstation back to the store for their techs to check out.... you'd think after all my years of troubleshooting things, I would have done a more logical step-by-step approach but hey, without these reminders of our own fallibility we might get too full of ourselves...? Even got my wonderfully supportive girlfriend to brave rush-hour traffic across town to get the machine back to the retailer while I got some work done.
So. Without CCC running, the fan was making revolutions as if the gas pedal were jammed right through the gods-damned firewall. A more accurate description might be the constant roar that lulls you to sleep on a 747, a roar which I am guessing is engine thrust combined with drag. In retrospect I am actually impressed by this, as it tells me that the card's fan is capable of generating quite a lot of airflow if necessary. And I ponder in wonderment that CCC installer is smart enough to know not to install CCC when the card is not properly plugged in.
With the card installed properly and CCC running, the card has been VERY quiet and we couldn't be happier.