I have to say I'm not too impressed with the Dell, so I would recommend that you go over the detailed specs of your motherboard, CPU and RAM capacity and see how far you can go with your current desktop. Using the Dell as reference you could conceivably keep the desktop running for another 2-3 years.
the laptop will lag like crazy even playing games like minecraft. I have an alienware 15r3 and That runs minecraft with optifine at like 110 fps only, that laptop wont do good, idk what to tell you man if you want to play minecraft upgrade your cpu which might mean that you actually have to get another mother board aswell. try to save up if you can. If not don't buy that laptop because you will just waste your money if you think you will be playing games on it. hey this is just a suggestion you don't have to listen to me but I think you should save up a couple more hundred and get a laptop with a i7 and gtx 1050ti which is good for gaming and portability