It's what switchable graphics are about. You have integrated graphics in the CPU chip for when you want battery life but not require graphics performance, and then 'optimus' (for NVIDIA) or 'switchable graphics' (for ATI) will turn on the dedicated graphics card automatically for games and such.
So, you get a lot less power draw on the internal graphics at the cost of poor 3D performance, and powerful graphics with the dedicated graphics card at the cost of poor battery life. You just can't get both at the same time
That's as far as technology is capable at this moment. While you play games on a switchable graphics setup, you'l still have relatively poor battery life (about 2 hours), but you get much better battery life when you don't need the GPU for browsing and watching videos on the go.
The previous technology (say, the Asus UL30VT) required the user to switch the graphics manually. The UL30VT also had a crude turbo boost that would overclock the CPU up to 33% also manually. You had to restart the laptop for underclocking or overclocking. Now it's just done automatically on the i5/i7/i3.
The HD 5650 has a TDP of 15=20 watts, which is pretty impresive for the performance you get. But 15 watts on a 55WH battery (plus CPUm about 20 watts) gives you under 2 hours battery life.
With the integrated GPU and the underclocked CPU alone and the dedicated GPU turned off, you get about 12 watts consumption, which would give you 4+ hours battery life.
The 4570 TDP is about 10-15 watts, so you get a bit more battery life for gaming but don't expect miracles. However, the 5650 is over twice the performance, which makes the 5650 very attractive.
The GT335M is just under the 5650 for performance / TDP, and the 320m is just above the 4570 and the 310M. That should help you make a decision.
