So, I want to download daggerfall but will DOSBox slow down my computer (since its an emulator) when it's not running????

also, in the daggerfall intructions it says something about making the memory size 32, but I'm running vista-64, so should i make the memory 64???????????????>

No and no.

Get a GUI front end for DOSBox, like D.O.G or D-Fend Reloaded (easily googled). This gives you a windows way of using DOSBox. It's a ton easier. I know DOSBox like the back of my hand, but I still use D.O.G. as I have 30 DOS games on my PC! Remember with an emulator you are setting it to emulate what the game wants. So if the game wants 32mb then you use 32!