Hello,
For a majority of people, The Sims 3 works absolutely fine on Windows 8. But for a small number of people there can be issues, often caused by program conflicts, however this can usually be dealt with using compatibility modes.
If it really must be an Alienware PC, I would recommend getting the one with the GTX 750 Ti as a minimum if you're wanting to play the game at the highest settings with multiple expansion packs. Depending on the resolution, the more expensive models should theoretically be slightly faster, but the GTX 750 Ti is a strong graphics card choice.
I personally do not like Alienware as they are often over-priced for the parts they offer. There is perhaps a better computer for cheaper from a different manufacturer, as well as other options in terms of custom-built computers.
Are you set on getting an Alienware PC?
I hope this helps and keep simming,
Cody