Well for games and video yes indeed, a huge difference, for general windows stuff I would think it would help but I can't say how much exactly. Like I say I think you will find it runs very smoothly even on the integrated video. I have Vista Ultimate running on a laptop with 4 gig RAM, a much less powerful CPU then you are getting and integrated ATI video and it runs very smooth - even with Aero enabled. So I suspect what you are getting will do the same at least.
It helps to have a self-built PC that doesn't have all the crapware that is pre-installed on brand PCs. More often than not this is what makes some Vista PCs feel sluggish, and XP machines too for that matter.