Before running any tests download and install Rivatuner and use it to keep an eye on the temperatures. DO NOT benchmark a HD4870 without this safeguard in place!
The most savage test (graphics) I know of is Furmark (google it) or Folding@home.
You can change fanspeeds through either Rivatuner or the CCC by unlocking the `Overdrive` feature and setting a fanspeed there.
For normal gaming use I find 35% to be about right but for extended testing, 45% might be reguarded as `minimum safe`
The card can run at over 110C but it`s an expensive item to fry so I`d say anything over 90C is too much, but no cause for panic.