My first question is: "What the hell are you doing with all that disk space?!?"
That's OK. I really don't need to know. At any rate, you are correct that you can't just add the second 200 GB drive in RAID 0 without wiping it. RAID 0 works by alternating the writing of each stripe (where the stripe size is some multiple of the sector size - the size of which has a tremendous effect on your RAID's efficiency) over the two disks. (I.E., stripe 1 goes to disk 0, stripe 2 goes to disk 1, stripe 3 goes to disk 0, etc.) So, once you've establised the RAID 0 (striped) drive set, you'd have to reload the data. (For RAID 1 - disk mirroring, you could of added a second drive and brought them both into a mirrored set and copied your old to your new without reloading. (I've actually done that.) That's the purpose of RAID 1, to write identical data to two drives. But you are trying to get more & faster space, not redundantly backed-up space.) I suppose it's possible that the controller could write every other stripe from an older drive to the new drive in the array, but I doubt that's supported. I think you also have to format the RAID drive in Win XP after it's set up, so it's probably a no-go. Maybe you could buy a third 200 GB drive, use it to hold your 200 GB of data, set up the RAID drive, load it, and return the 200 GB drive because you don't like it. (On the other hand, it sounds like you'll need in in a few weeks anyway.)
Keeping a separate (non-RAID) boot drive is fine, but you may want to move your swap file to the RAID drive. (Although file servers don't tend to have a swapping problem depending on the number of users, so maybe there's nothing to be gained there.)
My onboard Promise RAID controller supports either JBOD or RAID (0 or 1). I peeked at
http://www.highpoint-tech.com/ -> Support -> General FAQ - RAID IC and found
Question:
I have no need for RAID at this time. Can the RAID controller support single drive configurations?
Answer:
Yes - RAID is optional, not required. The RAID controller is capable of supporting RAID and non-RAID drive configurations.
Question:
Can the RAID controller support multiple arrays, or single disk and RAID configurations simultaneously?
Answer:
Yes. The RAID controller can support multiple RAID arrays, as long as enough free channels are available.
The controller will also support single drives configured alongside a RAID array.
They don't mention any IC part numbers here, but it sounds like you could hold off on that card. For the Promise controller, I have to load different Win XP drivers to use RAID vs JBOD, so I don't think it can do both at the same time. I'll mention this, but I'd guess you already knew: it's best to put each drive in a RAID (0 or 1) pair on a separate IDE channel for best performance. Otherwise, the two drives are fighting for the channel (which is probably more true for RAID mirroring then RAID striping). If you run out of channels, the separate RAID card is probably the way to go.
You may want to search out articles on setting your RAID 0 stripe size. In the case of Promise onboard controllers, you can't set the stripe size unless you flash your MB with a hacked BIOS (which are plentiful for ASUS MBs). It's one of the things Promise "holds back" from MB manufaturers. It sounds like Highpoint does not do that.
-cap