You didn't say how many people will use this server, what kind of data it will hold, what kind of performance the users will require, etc. Without knowing it, giving a good advice is impossible. The two answers above, for example, could be good in one kind of situation and end up wasting your time and money in another.
Still, it's possible at least to steer you in the right direction.
First of all, regardless of your goals, set up RAID: RAID1, if you need something very basic, RAID5 if you need a whole lot of storage and don't care about high performance writes, or RAID10 if you have money and need fast storage.
Then look at your utilization to choose between SAS and SATA. If you have lots of users, or have few users, but their apps access lots of files simultaneously, or need high sustained transfers, or need high reliability - get SAS. If none of this applies - get SATA. This includes a new controller (IDE and SCSI are obsolete - I would even guess that your computer is not too speedy by the 2009 standards).
You will pay about 3x-5x more per unit of storage with SAS compared to SATA.
And now - forget everything I said and go get yourself a Drobo that would fit your needs.