I like to keep a small (8GB) partition up front for windows, then put the rest of the stuff on a 2nd. (I have a 80GB WD800JB).
I do the same on my systems.
An 8 gig C drive for OS, apps and games... the rest as D for data.
This works well for me on 40, 60, and 80 gig drives.
Doing this makes format-reloads very easy... if your OS goes, just delete the C partition and reinstall -- all your data on D will be safe. It also helps prevent program/data disk fragmentation.
If you primarily game, you may want to make your C a touch bigger to make room for big game installs (16gig is more than enough). Actually, 8 gigs is enough for a few good game installs -- but I dont know how many large games you will want to install.
Or maybe make a separate partition for apps and games apart from the OS... but thats seems like overkill.