What case do you have Gothch1ck? and how many fans? I notice our systems are fairly similar. I've had no problems in Fallout 3 for example (or far Cry 2), though I don't play the sims.
First I would check the basics:
Does your mobo need the bios flashing to support your processor? have you done that?
Have you stress tested your CPU with prime 95 and your memory with memtest86?
Have you installed up to date motherboard and graphics drivers?
Have you stress tested the GPU with something like rthdribl or furmark?
Do you get any blue screens or other errors?
Have you checked load and idle temps for both CPU and GPU?
900mhz isn't stock speed for the 4890 so I'm guessing the card is pre-oc'd, if you are getting artifacts try lowering the clock to stock (850Mhz) and 3.8Ghz memory, if that gets rid of the artifacts then send the card back.
Additionally I suppose windows 7 could be to blame, i'm running XP32 at the moment. I'm installing windows 7 in a couple of weeks so I can report back then if I'm experiencing any oddities.
As for your bright monitor, bear in mind that the recommended setting for an LCD is 120 cd/m2 which is likely to be a fraction of your monitors capacity (most monitors have a default of 300 - 500). depending on your monitor the correct setting is likely to be somewhere between 20% and 50% brightness so don't be afraid to turn it down.
Finally an echo of my first post, turning everything on/max in CCC is just asking for trouble IMO.