a gig of ram is just a luxury pretty much. the only time ive ever used that much ram (or needed) is when i do simulations in matlab that are pushing 1kx1k matrices.
256 sticks overclock better, but i doubt you will see much of a difference in multitasking speeds between the gig and the 512. i do the same thing on my box im currently using.
right now im folding, using aim, streaming music across our intranet to 46 connected users, playing winamp and using mozilla composer without any lag in performance. total ram usage is 347meg of the 512 and 19meg in my swap partition (fat 16 of course).