No, I wouldn't sell it. New specifications come out all the time. Does your present one do the job? (I'm aware that you have problems with the speed of your Inernet connection, but changing your computer won't fix that.)
I tend to keep my computers for about 5 years before upgrading or replacing them. I may decide to upgrade my Mac Mini soon (which is now over 5 years old) but only becasue, being one of the old models, it won't run Snow Leopard so I can't easily use it for iPad development.