One gig is a better price/performance point, right now, for most new rigs, VERY serious gamer's, and photo editing, aside.
Will four gig of RAM be better than two? Only when you have apps/games that really need that much. Other than video/photo editing and HUGE regressive analysis, I don't know of any apps that benefit from it.
If you mean to use this system for 5 years or more, I'd suggest whatever you get, make sure you can double that when you wish, in the future. If you have 4 memory slots, then 512 X 2 would do for 99% of the games/apps now, and allow you to expand to 4 X 512 (2 Gig), whenever you felt it was needed.
If you plan on holding onto the system for longer than 5 years, I'd suggest going 2 Gigs now with 2 X 1024Mb, now, which leaves the easy upgrade to 4 Gigs, available to you, later.
More memory is usually better, but every part of a RAM chip MUST be "kept alive" with a signal. Having WAY too much memory for your work/play will actually SLOW a system down, (a small amount, to be sure), because these signal sweeps have to be worked through, for more memory, and that takes time, and energy.
Adak