Yes its fine to mix memory, it just lowers the speed of the faster one to match the slower one. Currently I'm mixing a 3200 and a 2700. Good deals? I'm not sure, I usually look on www.newegg.com and www.pricewatch.com
If your going to upgrade your memory, dont go with a faster one that you already have since you will be paying more money for memory and you wont be getting faster speed. But yes, it is compatable.