need advice on testing & modifying themes

[ponygirl]

need some expertise here...I want to make some changes to my blog. I have a wordpress theme that I would like to customize, (I want to go from a 2 column to a 3) but obviously I don't want to be switching themes back and forth on my live blog while I do that.

what's the best way to do this? Install another copy of wordpress in a subfolder as a test blog? I hope this isn't a stupid question but it's been so long since I installed the first one that I feel like I'm starting over. Which I guess I am

thanks in advance for any suggestions!