Budget build for all expansions
CPU : Core i3-3220
M/B : Asrock H77M
RAM : 2x4GB bus 1600
VGA : Radeon HD7770
HDD : up to you 500GB or 1TB
Monitor : 1366x768 resolution
OS : Windows Vista or 7 or 8 64bit
Performance build for all expansions
CPU : Core i5-4570
M/B : Asrock H87M Pro4
RAM : 2x4GB bus 1600
VGA : GTX650Ti Boost or Radeon HD7850
HDD : up to you 500GB or 1TB
Monitor : 1920x1080 resolution
OS : Windows 7 or 8 64bit
Ultimate build for all expansions (You must have knowledge about overclocking.)
CPU : i5-4670K
CPU cooler : at least Coolermaster Hyper 212x , if heavy overclock you may consider liquid cooling such as Corsair H100i or Coolermaster Seidon240M etc.
M/B : Asrock Z87 Extreme 4 or Biostar Z87X 3D HIFI
RAM : 2x4GB RAM bus 2133
VGA : GTX760 (I see gamescom use HD7950 to show playing SIMS4 , GTX760 is a bit better , I think it can handle all Sims 4 expansion.)
HDD : up to you 500GB or 1TB
SSD : Samsung840Pro or PlextorM5Pro or OCZ Vector or OCZ vertex450 or Sandisk Extreme2
Monitor : 1920x1080
OS : Windows7 or 8 64bit
What to overclock ?
CPU : at least 4.5Ghz
RAM : 2800Mhz
VGA : don't necessary
Too bad Sims 3 is using directx9 which result in game TOO RELY ON CPU SPEED (Singlecore performance).
Overclock CPU can help increase framerate and reduce loading time.
When high speed SSD combo with CPU overclocking , it can ultimately reduce load and save time.
http://www.youtube.com/watch?v=2opucq6nF5w