GPU OC is NOT limited by your RAM
CPU OC is limited by your RAM, only IF you do 1:1 overclocking. Most motherboards these days have a Memory Divider, to bring your RAM to lower speeds and then it shouldn't limit your CPU OC.
Now I tell what i said earlier again - it does not depend on the manufacturer, but as Undertaker rightly said, it depends on the chip used in the RAM module.
Corsair Value Select uses standard chips which do not OC much and can run stable and fast only at stock speeds.
If you were to get Corsair XMS on the other hand, higher grade chips are used in the those memory modules, you would be able to OC to much higher speeds and then would it generally be much faster.
Same case with any brand - OCZ, Hynix, Kingston etc - if you buy the lower-end products, expect lower-end performance. If you buy the higher-end products, expect fast and stable performance.
The Corsair chips you are talking about for 3K are Corsair Value select, which means you can expect decent performance as long as its NOT overclocked. If you want high-end memory, try something else.