No, this is not a simple problem of my GPU being too old or me not having DirectX. And yes, I've searched for a solution extensively.
The problem seems to be that when I change the settings ingame, then exit and start it up again, I will get the "Unable to find DirectX 9.0c compatible adapter blarg blarg blarg". This should not be, as I have a system more than capable of running the game, and I have DX11 (according to dxdiag, I'm pretty sure my card only handles 10, but yeah). I've reinstalled my drivers several times, and my chipset drivers, but the problem persists. I emailed customer support but got a generic solution which, of course, did not work.
Motherboard - MSI NF750-G55 (nForce 750a chipset)
CPU - AMD Phenom II X4 955
GPU - Geforce 9800 GTX+
Hard Drive - 640 GB Western Digital Caviar Black
RAM - A-DATA Gaming Series 4GB 240-Pin DDR3 SDRAM DDR3 1600 - Model: AX3U1600GB2G9
Operating System - Windows 7 Home Premium 64bit
(The game is "The Sims 2 Double Deluxe", just to specify, so it is fully patched.)
Try downloading the latest version of DX9c from microsoft, DX11 is not fully backward compatable with DX9, this will not overwrite your dx11 version, you will have both on your system. Thats only thing i can think of.