Right, you want a drive not more than 5400rpm. The original DMS drives were 4200rpm Toshibas (at least the two I had were). It's only the rubber "bumpers" that might not fit perfectly (but they will fit), as they were tailored for the stock drives. Any 2.5 inch ATA drive will fit in the DMS shell.
I'd suggest getting a 160GB drive, since they cost only 5 to 10 bucks more than the 80GB drives.