HD5570 Overclock + lower temps

Be careful overclocking to the max. Check if you don't experience artifacts by running Furmark and atiTool. If it's still stable after an hour or so you should be good. If it's a new card it could be correct that it will lower in temps after time because the thermal paste needs to heat up and reach it's optimal efficiency over time.

If you don't experience a performance increase your card could be overclock protected.
You could go even further if you have a sufficient PSU. Go and see techpowerup.com. They have a detailed tutorial on how to flash and edit your GPU bios so you can up the voltages a bit and increase the max overclock. But do so at your own risk! read...read...and read... .

these readings should be correct but for a more accurate and detailed reading you can get AIDA64 which is the follow up of Everest to get a full report on all your temps and system specs.

and to conclude...overclocking your CPU would make sense if you're using CPU hungry programs but as for gaming you could be bottlenecked by your GPU. so in that sense it would not make a very big difference.

hope this helps!
Jeffrey.