There VERY LITTLE overhead for overclocking.
I do NOT agree that it's better to overclock yourself. Overclocked cards often have better heatsinks such as the Vapor-X technology by Sapphire.
I just don't think it's worth manually overclocking to get only a 3% stable overclock.
UPDATE:
I was about to say the HD6870 is a great card (and it is). However, I just went back and spent some time looking at the latest cards. I high recommend this:
NVIDIA GTX 560 Ti
http://www.tomshardware.com/reviews/nvidia-geforce-gtx-560-ti-gf114,2845-15.html
*NOTES:
1. It was quieter than other cards in its class
2. It was back and forth between it and the 6870 for games
3. SCALING is better if you think you'll buy another card in the future
4. NVidia is better at driver updates
5. Tesselation is better on NVidia DX11 cards
6. CUDA support
7. PhysX support
Hands down, this is the card I'd buy today. It just didn't exist last year when I bought mine. Obviously they've sorted out the heat issues.