You know, you could always buy the larger drive and only partition off 20-40 gigs on it. You can leave the rest alone, always able to expand it later if necessary. I would actually suggest raising that number to about 80 gigs if you're going to install Vista, but like I said you can leave only a system partition on the disk and not allocate the rest. This gives you an added benefit of minimizing head movement and allowing for moving the partition space in the event of bad blocks in the future.
Just a thought. Plus you'd save money for not having to buy more expensive drives*.
* I'm actually looking forward to the ability to move all paging to solid-state drive space at some point. Total solid-state drives aren't feasible (economically) yet, but enough for paging is.