Catalyst Control Center gives GPU clock = 500 Mhz for Radeon 4870 HD

Thats the 2d clock mode its a power saving feature... although how well it works is to be questioned as it often can cause crashes...

Anyway its completely normal and as soon as you start up anything intensive it will go to its stated clock value

The memory clock on the 4870 has to stay the same because there are image issues if the 2d clock and the 3d clock are different... artifacting can happen but more likely... and more common is the side effect of the screen flickering if the 2 clocks are different for memory