Hi...I have an old sony vaio...that takes a umda66 hardrive...(neverheard of that one) it has 2 slots...so i can add another one....but what kind do i install.....IDE?SATA? .....

umda66 is the same as ATA/66 which should be a plain old IDE laptop drive (2.5"). What model vaio? You might want to google and see if there's a bios limitation on size.

Pretty old I would say.

It started with UDMA 33 (about 2 to 3 Gb), then 66 (about 30Gb) , then 100 (about 40 to 80Gb) and the current standard is 133 (up to 500Gb). The number is to signify the maximum transfer rate.

Then we have Sata 150 moving to Sata 300 now.

This is a sony vaio...PC.not a laptop will an IDE harddrive work...and the computer is old but old to me is 6 years.....

Yes you can add another disk.

I expect a mobo of 6 years should be able to handle ATA133 and so you can just go and get any 3.5" internal IDE or Pata hard disk in.

Older mobo may have bios limiting the size of the hard disk but a 6 years old machine should not be one of them.

You can hook the new disk onto the same ribbon cable but a separate one with 80 wires is recommended for connecting to the second connector on the mobo. The 40-wire ribbon cable can't do ATA133 which you will find a lot fast than UMDA66.

If the hard disk come up alright you may consider migrating the operating system in the old slow disk to the new fast disk. This thread may be of interest to you.