At least 3 companies make 640GB 2.5" 5400 rpm drives. Formatted they usually end up around 500-550 gig. Western Digital just came out with a 1TB 2.5" 5400 rpm drive, for those of you looking to maximize space on your PS3.
Can’t go wrong with a western digital caviar green
The HDD height on the 1TB and 750GB drives are actually 12.5mm so it might be too big for the PS3. But you could always go for a 3.5" five if you don’t care about getting a power supply for it and being outside of your PS3.