Hey there.
Partitioning is really not a bad idea if you don't need the whole space. It's easier to organize things, plus you can always reformat the non-OS partition if you decide that you want to at some point. Other than that you could also migrate the OS partition at some point (if you need to) without the partition with the games (you can't do that if everything is on a single partition).
On the space needed for your OS and Program Files, this is really up to personal preference, ability to manage disk space and what programs you'll be installing. Some people can manage an OS partition of Win 7 with just 60GB, however I'd recommend that you make it at least 100GB if you plan on installing a lot of storage hungry programs and to keep some space for updates.
Make sure that you have all secondary drives disconnected from the motherboard during the Windows installation process. This could save you from some booting issues in the future.
Hope that helps. Please let me know if you have any questions.
Boogieman_WD