I did similarly. I think the main considerations are how much turnover you have in memory, how often you are going to back-up, and how far back you want to back able to retrieve files. If you don't want anything too old, like me, and just like having the past week, you don't need a whole lot more than is on your comp hd now.
It is too back that Time Machine isn't integrated into typical storage somehow. At the current I have everything in one partition (including everything not on my Powerbook, such as rarely listened to music), while I also have everything on my computer doubled in Time Machine. I guess as long as the drive doesn't fill up you can use TM to hold files like these, but if it does then you will start to lose all that old stuff that you want to keep somewhere but not on your comp's hd. Maybe just a way to mark certain files or file types as "never make room by deleting these." 2 cents.