The answer is easy: both :)

When the first DDR memory was introduced, the memory chips continued to run at 100 MHz, 133 MHz etc., whereas externally data was sent at double that rate. Hence the term double data rate = DDR. Since advertising likes higher numbers, this was sold as "200 MHz DDR", "266 MHz DDR" etc.

In your case 1000 MHz should be the physical clock rate of the memory chips, corresponding to the 100 MHz in the example above. The 2000 MHz corresponds to the DDR data rate. However, your card should have GDDR5, where the external data rate is even higher, so this should actually be "4000 MHz". You'll see the latter in current advertisments.

MrS
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Scanning for our furry friends since Jan 2002

This GPU-Z issue was also experienced at another GPU project!

Normally you don't need GPU-Z to be running, it's just a reporting tool. MSI Afterburner and EVGA Precision are the programs that allow you to make adjustments to the fan speed and GPU clocks. Just use one of course.
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FAQ's

HOW TO:
- Opt out of Beta Tests
- Ask for Help

And you don't even need to keep these open all the time.

Generally any 2 programs which monitor the GPU and hence poll the sensors frequently can cause trouble.

MrS
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Scanning for our furry friends since Jan 2002