Looking to build a server for my company. I have an amd cpu, phenom 2 x4 i believe, tower, could pull up specs later when i am near the computer. Want to install a linux server os on it, prob. ubuntu because I am new to linux. The server will host my companies website and a database that will be accessed by 2 - 10 users. Will I be able to get by with this old tower, or do I need to look into getting a "server" to accomplish what i want to do? Also, would I need to purchase anything extra to set up a raid, or should be able to do that since I currently can and what RAID should I be using?

First, if you are 'new' to linux, do you really think your company wants you to learn linux on its production website and database? No, I don't think so.

RAID is important for most physical servers, 3 disk RAID 5 is pretty standard but raid1,6,10 are also viable options depending on what the server will be doing and how the database will be used. Way to many details to go into on a single post.

My opinion? Install CentOS 6 (a clone of Red Hat Enterprise Linux, but without license/support fees). RAID? Optional, but more reliable than LVM. I'd recommend a 4 disc RAID 5 array with hot-swap capabilities. That way, when one disc goes south for the winter (a technical term for failure), it will continue to operate until you replace the drive, and then rebuild the drive automatically, returning you to full operation without much effort on your part, other than swapping out the failed drive.

The others have hopefully answered some of your questions, but just to throw in my two pence.

An AMD Phenom processor should be more than capable of supporting 2-10 users, even if they are concurrant. (However it all really depends on what your server is used for, as you mentioned a DB then what is your RAM like?)

In terms of what RAID system you use, is dependant on what hardware you have available and what value the data you are storing is. In practice, you are likely to be running a software RAID which can be configured easily with the use of Ubuntu server and therefore the main question to ask is what would happen if you lost the data.

If the data is "mission critical" then a RAID1 system is the way to go, this shall ensure an exact copy of your data is stored, and so won't require recovery with the use of one of the other drives.
All in all, RAID is a must have if you are storing anything of use. Just don't go for RAID0 (which in my mind shouldn't really be called RAID at all). If one of your drives bricks it (dies, goes bang, stops, explodes in a shower of sparks) then you won't have a fall back.

One question that no one has seemed to ask.. Is this going to be used for an Intranet or Internet web server ?? If you are going to use it for an Internet server, I would recommend against putting the database on the same server.