I just upgraded memory for 3 computers that I use daily. Processor speeds have not improved that much recently. Currently, in my field, there is not enough software written to take advantage of the additional processing power offered by dual core chip designs. Adding a couple gigs (yes, gigs) cost me about $80 per machine. The improvement was immediately noticeable.

The listed "speed" is not as important as compatibility with your computer's chip set and front-side bus speed. If you want a free scan of your machine, you can go to 'crucial' memory's site and use their on line tools. Then go to the web and find compatible memory sticks as cheap as you can. If you browse around, you may find "refurbished" memory dirt cheap. Just my .02.