Get 7200rpm. It is the interface that matter, not the rotational speed.
Check if your motherboard supports (P)ATA or SATA and get a drive with one of those interfaces.
If you want to go external, you need an external hard disk enclosure (again, (P)ATA(100/133) or SATA). These enclosures mostly connect to your USB port.
Using the disk as an external disk will be slower, especially if you don't have USB2.0 (eg USB1.1).
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....