The best we've been able to find and comfortable suggesting is:
CUK HP Envy Gaming Desktop = Starting at $1,649.99
http://cukusa.com/hp-envy-gtx-980-ti-oculus-rift-vr-ready-gaming-desktop.html- Intel Core i7-6700K Quad Core Processor 4GHz
- 16GB (2 x 8GB) 2133MHz DDR4
- Nvidia GeForce GTX 980 Ti 6GB GDDR5
- 2TB 7200rpm Hard Drive
- 500W Power Supply
You can get this configured to your liking. For example, if you want a solid state drive. They can add one for you.
This will play both Sims 3/4 and/or any other game you want at highest settings.
Please note: With Sims 3, please keep in mind due to the game engine's quirks the game still suffers from micro-stutters and periodic slow downs. There is not a lot you can do to fix this.
Also the more content you add to the game, like custom content, game mods and store content the more the game will lag. This is due to the way the game handles additional content. Again, not much can be done to speed that up unfortunately. But it is far less noticeable on a desktop.