Macbook pro 15" Battery life

Hi,
Im getting soon a 15"macbook pro, the 330M with 512mb version. I was told that the GPU changes automatically from the intel integrated to the 330M, but I was wondering if I could use JUST the 330M, I mean, to switch off the intel integrated completely . And if I can, can you guys tell me how much of my battery life I will lose ?
I am asking this because I went to an Apple store and tried a few 15" new models, and the intel integrated is all way sluggish and choppy.
Thank you in advance!

But the option to disable the integrated one through system preferences , like in the old macbook pro with 9400M+9600M GT , isn't present anymore ? I remember we could choose between battery life and high performance. Is this still available , or only with this software you recommended I can do this ?

THats what I really want to know, How long the battery will last if I use ONLY the 330M. And if using the 512mb version will cost me more than the 256 mb version. If it does, how much ?
I know that with the automatic switching it can last for 8-9 hours , as Apple claims. My fear is that using only the 330M will cost me more that 2 of those 9 hours, and reduce significantly the battery efficiency.