Just got a server and start something new..

Before I'm only with some cheap servers and small users, do not have any experiences about the max numbers of user.

I got ..

P3 866*2
1G ECC SDRAM
36G SCSI(10000 RPM)*2
with in 2U

How many users and Bandwidth it can handle....

Depends on the content you will be serving.

If you're serving purely static content, that server should be able to serve somewhere around 10-30 TB/month. Otherwise it depends entirely upon the script load.

..and most importantly like what the others had already said, what content will be serving i.e pure html or server side scripts also?