@wankawitz
If your seeing artifacting the 1st thing you need to do is determine what's the cause of the problem. Again using a program like GPU-Z and ATT you should be able to get all the info you need. If you want you can post a screenshot of this.
These are with stock reference 4870.
Load 4870 1GB
Idle 4870 1GB
If you feel that the card is really can't handle the stock clocks then it needs to be RMA. You should always go to the place you bought it 1st to see what they can offer.
As far as cooling goes these cards do run hot by default. There's 2 reason for this.
1) the idle clock sucks on these cards ( this can be changed via ATT )
2) the fan speed are to low by default ( increase this during load )
Also I don't recommend using Furmarks just, because it pushes the VRMs to hard on these cards.