I like how you numbered your questions, but I'm answering out of order.
4) For a single client on a 100mbit LAN, an old Pentium will do the job. The whole 100mbit/s probably won't be used most of the time. For mp3s, you won't notice the difference between 100mbit and gigabit until you watch 300gb of data go across the LAN at once. Getting proper gigabit speeds takes a fair bit of effort in getting the buses and hard disks fast enough, and requires a modern (Pentium 4 era at least) processor.
http://forums.overclockers.com.au will help you better than I can; perhaps try the Internet/Networking forum there.
3) I would recommend RAID1 as a backup precaution if you can afford twice as many hard drives. I have seen it to work well as a safety net. I have seen it save vital data and the owner of the data was very pleased. You could try two different brands of hard drives to partially protect against bad build quality.
RAID0 is only useful if you find that hard disk speed is what is limiting your transfer rates (which won't matter if you're only serving up mp3s one-by-one or streaming audio). Since RAID0 arguable introduces a number of extra riskiness, I would suggest that if you use RAID0, use RAID1 as well.
Do you have 300gb of data to store? If not, I would suggest two smaller hard drives with some kind of RAID.
I would also suggest that you keep the gigabyte:dollar ratio in mind when purchasing hard drives.
If you are going to have a 300gb database, you will want to check out the man pages for the ext3 and reiserfs tools to optimise the filesystem for large files.
2) Are you planning on storing mp3s in a database? I'm wondering why you would do this, apart from maybe searching efficiently. Can you please tell what software you plan to use for that?
For the database itself, I recommend MySQL. It has lots of support and is open-source. When people use proprietery databases, it is usually for guaranteed support, brand name, and scaling. You do not have a scaling problem: despite the potential 300gb size of your database, scaling problems come in when many users start making many requests. A single user will not cause enough load to warrant anything other than the MySQL you can download for free and set up using free manuals.
-pcweirdo.
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