The MemoryGel implants are sized according to cc's and should correspond to to volume stated in the saline implant in cc's as well. I do not know the style of the saline implant you are wearing, and the projection of the implant, or the degree of projection produced by capsular contracture can greatly change the perceived size of the implant.
You will most likely have to consider a 500cc+ gel implant, and also consider that it takes about 100cc difference in an implant to produce an end result that appears larger than your current augmentation. You probably will not notice a change with 50cc, and may well want more than the 100cc increase.
By Chicago standards a 600cc gel implant is quite large, so work carefully with your surgeon and review prictures of what you would like your result to be. Your surgeon should have the burden of selecting the best implant for you with your pictures and goals in mind. If the result you look at each day is what you had hoped for, what is on the inside makes little difference.
Best of luck,