IMHO, besides achieving 1200x600dpi resolution, adding additional RAM also allows you to print at the 15ppm max speed, and minimizes system interrupts to your PC (which might not be too noticeable if you have a fast enough PC, but on a slow PC you probably would notice it). With 2MB buffer, the printer has to interrupt the PC every time it finishes printing a 2MB chunk, to get the printer driver to send it more data. This is especially relevant when you're printing documents with many pages, or graphics intensive documents.
By the way, FYI... the rebate for this deal is processed by Young America which is the best/fastest rebate processor out there, which makes this deal even hotter.