Good choice with the AMD Opteron based server. Did you mean 2x Opterons? That would be ideal for a database server. The AMDs will give you better performance than Intel's equivalent, and will run cooler and last longer, plus you can compile in 64-bit support in the linux kernel. Just make sure you pick out registered DDR Ram. It is a bit more expensive, but is more reliable and stable and is required for Opteron boards. Still cheaper in the end than an Intel based system. I agree the WD Raptors would be a wise choice for hard drives, but still stripe them in RAID 0 config. If this server is just doing database work, then storage should not be an issue, even a database with a huge amount of entries dont take up too much space, they are quite efficient. And to answer your other question, no another drive cannot be added to create a raid0 without losing your data. You will need to back it up on a separate drive or on another server with lots of temporary storage.
Hope this helps and good luck
Edit: On a lot of new motherboards (if you are using onboard raid support) you can add two more disks and go from Raid0 to Raid0+1 without losing any data, it will just take some time to construct the mirror