I have a MacBook 13inch 2.3 I got last spring and I don't want to slow my Mac down so is it possible to get bootcamp downloaded onto my Mac but using an external harddrive so it doesn't slow down or mess with the performance of my Mac?
I'm supposing you mean install the bootcamp partition on the external HDD (Since the bootcamp software is pre-installed in OS X)? Well no you cannot do that as Windows will not boot of an external HDD, but installing it on your main Hard-drive won't slow down your Mac at all as it is on a separate partition to OS X. And if after you install it you no longer want it removing the bootcamp partition is quick and easy.
Similar question: I'm planning on buying an external HD, in part because I'm running out of room on my Bootcamp partition (SWTOR took up a lot of space). Is there any way for my various windows games to play nice with the HD?