These are the 2d clocks your seeing the 500mhz's..
When you fire up a game, it will go streight into 3d clocks which is 750/900.
If you save a custom profile go to:
C:\Documents and Settings\<your pc name thingy>\Local Settings\Application Data\ATI\ACE\Profiles
And you can Edit the 2d and 3d clocks in there if you wish, no need for 3rd party applications really.. you can edit voltage in there too..
I run this:
<Feature name="CoreClockTarget_0">
<Property name="Want_0" value="30000" />
<Property name="Want_1" value="75000" />
</Feature>
<Feature name="MemoryClockTarget_0">
<Property name="Want_0" value="30000" />
<Property name="Want_1" value="90000" />
</Feature>
<Feature name="CoreVoltageTarget_0">
<Property name="Want_0" value="1050" />
<Property name="Want_1" value="1250" />
</Feature>
<Feature name="MemoryVoltageTarget_0">
<Property name="Want_0" value="0" />
<Property name="Want_1" value="0" />
<Feature name="CoreClockTarget_1">
<Property name="Want_0" value="30000" />
<Property name="Want_1" value="75000" />
</Feature>
<Feature name="MemoryClockTarget_1">
<Property name="Want_0" value="30000" />
<Property name="Want_1" value="90000" />
</Feature>
<Feature name="CoreVoltageTarget_1">
<Property name="Want_0" value="1050" />
<Property name="Want_1" value="1250" />
Basically, where my values are 30000, thats the 2d clocks which for you will say 50000 hence 500mhz in 2d mode, i run 300mhz in 2d mode which is lowest you can run, so it saves the life of the card when your doing nothing on your desktop, and it lowers tempuratures, and then where it says 75000 and 90000, them are your 3d clocks
Have a play around with it, just don't go silly and try overclocking too high, but its definately worth sticking the 2d clocks to 300mhz for each core
EDIT : just found the old link from few years ago, explain's what im on about with the gpu speed editing via the profile thingys
http://hardforum.com/showthread.php?t=1337485
have a good day!