Partitions for new HTPC

Hey everyone, I just ordered the parts to put together a HTPC, including a single 1TB Western Digital hard drive. I am planning to install Vista or possibly Windows 7b (along with Media Portal). On all the computers I've built (last one was 4 years ago), I've used a small partition for the OS, and unless there is some new, prevailing technique, I plan to do the same.

My question is, how large should I make the OS partition for this system? Should 10GB suffice, or should I go up to 20? Thanks!

There is no compelling reason to have a OS only partition.

Vista will use in the neighborhood of 15 gigs. Win 7 will use about half that. I would make it large enough to handle all of the apps you intend to install and have plenty of room left over.