It shouldn't matter, but it does.
The board doesn't have onboard video, nothing to disable.
Trust me, I tried over half a dozen different driver versions, from ATI themselves to the manufacturer's webpage itself... It starts the install then sounds an error, quits, and says "No valid hardware detected" or some such. There's definitely a hardware conflict going on.
Nothing can tell there's an X1650 installed on the system except the drivers that came with it, and those CLEARLY don't work right.
There are many similar stories with X1650s that I found with a google search. None exactly like this (with the sims and all) but many to show that the card has many problems on many machines. For simplicity's sake it's going back. Not worth the effort.
EDIT: P.S. I checked the PSU output, and it's enough. There's not tons of stuff in this machine. It doesn't have a power hungry CPU. It's only got 1 HD. The connector on the vid card was isolated (no other items plugged into it). It should have worked fine. It just didn't.