That's actually the exact drive I ordered on Saturday for my brand new Slim. I was considering 500GB, but decided I really didn't need that much space, so 640GB was obviously ruled out too. I won't be using my Slim as a multimedia device (that's the role my laptop fills), and I only wanted more...
For anybody seeking outrageous amounts of storage space for their PS3, Newegg's PM Shell Shocker today is a Western Digital 640GB HDD for $120.
And for anybody who needs them, here are tutorials for replacing the HDD in your Slim and Fat.