I generally use 70gb because it's definitely enough for Windows, Office, and Adobe software, plus plenty of games, and any other software you might want. I have run into the limit with 60gb, but never with 70. I am enjoying the rate at which hard drives are expanding, though, with a 320gb hd on my laptop. Next one I get in a couple years will probably have 500. Plenty for my programs, my music, my videos...
But enough about me, if you have a 200+ gb hard drive I would go with at least 50-60 gb for programs, knowing that if you ever need more space for either software or media you can generally repartition your space, but it is harder and takes longer to make the larger partition smaller.