What it says it that your memory is operating at 533MHz. See the third screen above - the memory is "dual pumped" so the 266.1MHz x 2 - = 533Mhz. That screen also shows that all 2560MB of memory is working.
The memory is working at 533MHz because that is the top speed of the 512MB memory - see the next screen- althought the 2GB memory is capable of speeds up to 667Mhz - see the next screen which shows the three possible settings that memory can use. Mixed memory always works at the speed of the slower memory.
If you mobo supports the higher speed of 667MHz, then taking out the slower chip may allow the larger one to speed up. Nothing on the CPU-z says what will happen - but if you don't have mobo specs you could just test it for about a 25% increase in top speed.
However, speed (bandwidtth) is actually only part of the "speed" equation - actually calling the MHz alone the speed is a bit of a misnomer. The timings address lags and times between operations. Your computer is now operating at 4-4-4-12 - see screen three above again- which is actually quite good. This is because both chips can operate with this setting.
Note that on the Kingston screen, under the 333 MHz frequency (which times two equates to the 667 MHz "speed", the timings are slower at 5-5-5-12 - or roughly - and this is only approximate - 20% slower. So even if the Kingston will run faster at 667 MHz, the speed gain will be mostly offset by the 20% decrease caused by slower timings. Note that the 5-5-5-12 timings are the same as the ones I reported above for the mislabelled 1 GB Hynix with the 667 speed. It is pretty common for basic mainstream memory - and timings typically increase (decreasing speed) as the MHz increases. That is one reason that newer faster memory is not that much faster. They have to increase the timings to let the faster memory stay stable. Over time they usually find ways to better "tune" the memory or manufacture it better and can reduce the timings, but usually not all the way down to the slower memory. I did however, just buy 800MHz memory on sale with the 4-4-4-12 timings.
Anyway, your memory will probably run about the same with one or both sticks and with both you get the larger memory so you are probably better off with both. And now you should know a little more about memory and your computer.
If you don't have it yet, you might want to download PC Wizard to learn more about your system.
http://www.cpuid.com/pcwizard.php