ATA is the regular, parallel IDE interface found in most desktops, as opposed to Serial ATA (SATA). The number indicates a speed rating, ATA-6 is backwards compatible with ATA-5. They have a 44-pin connector: 40 for data, 4 for power. Any of these should do. I'd recommend you pick a 5400 rpm drive; 4200 is just too slow, while 7200 may get too hot and require better cooling, which an old laptop doesn't have. The cost per gigabyte may be a good indicator of what's the best buy at the moment. The Samsung 80 GB looks like a nice deal at $60, same for Toshiba 80 GB; personally I'd go for the Seagate 160 GB at $140 - plenty of storage and a reputable manufacturer.