GTX 460 is very good I hear, I have a GTS 450 and it's great! Have a look at the AMD 5850 as well, it's the next generation of your current card as it were, and very powerful indeed. I reckon if you look around you can get them under a hundred quid as well!
I reckon to see a decent performance gain you're going to have go with a GTX 460 or a 5850. The 4850 is no slouch as a gaming card, so you're going to have to go pretty high end to see a decent improvement for your money. It would be worth reading up on the power requirements of the above and similar cards before getting a new PSU!
Q6600 power demand are 135 watts and above i have the same cpu overcloked in 3.30 Ghz With VX450w you will have only 396watts and in this amoud must devided to your Q6600 and the card stay in your 600 watts is better.
That may be so, but as stated previously, if you want to see a decent performance increase then you need to get a faster, power hungrier card. It wouldn't be great for you to spend money on a new PSU that was quite average and then be limited to cards which are round about the same performance as your current one.