Where did you get the 8400GS from?
If you mean 'Half Life 2 - Orange Box' (HL2OB), then a 8400GS is probably the lowest card that supports it. It should be fine for Sims 2 though.
49° C for that card under stress is about right I think, so no problems there.
Try to get a minimum of a HD4670 for HL2OB. G