What kind of hardware would i need if i plan on hosting maybe 15 personal sites small very low bandwidth and say 2 somewhat active sites say 20-30 people online throughout the day ?

Im new to this so im trying to learn some stuff just in case i ever need a dedicated server.

I doubt you'd need more than a 500mhz server with 128MB RAM. Although most dedicated server hosts don't even offer specs that low anymore.

One of our original dedicated server customers is on a PIII 500 w/ 256MB RAM and does around 10 million hits a month without a problem. He's approacing the upper limit of his current hardware, though.

I was thinking of building my own i did put together my personal computer.

What OS do most servers use ?

Depends on if you're going with Windows or UNIX.

For windows, there aren't really a lot of options...

For UNIX, the most popular OS here seems to be RedHat. Personally, I prefer FreeBSD. Almost all our dedicated servers run off it and we've found it to be extremely stable and secure.