I assume you're considering RAID0 to get more performance. Well, from my experience, using RAID0 indeed gives you a great increase in performance, and I am all for it. Actually, I must say that at first I was quite skeptic, because technically it only gives you better sequential speed, while access times remain the same. But in fact, in day to day use I was surprised what a boost you get from it.
However, and this is very important: for your mental health, **DO NOT** put anything on a RAID0 disk that is not backed up!! Beyond having x2 the risk of loosing the info (it is enough that one of the drives crashes) it is also much more complicated - and expensive - to recover a RAID0 disk.
As I see it, these are your options:
1. Have the two in RAID0. In this case, you *must* have another HD in the system for backing up your data, or otherwise back up to DVD in a regular basis.
2. Forget about RAID. In this case you can use partitioning to gain a bit of speed. Put at the beginning of the first HD a 20-30GB partition just for Windows+Apps, and at the begining of the second HD a partition just for your swap file (size it to about x2 your RAM). You benefit here because the partitions are both at the fastest areas of the HDs, and because you can simultaneously r/w Windows ans swap files. In fact, I would also have another partition at the begining of the second drive - say right after the swap parition - for temporary files. You can move all your TEMP directories to there and hopefully gain a bit more speed there.
3. This one is a bit more involved: most RAID controllers now allow having different RAID configurations on different parts of the same HD. What you can do is allocate a 20GB partition at the beginning of each drive, and have only these two partitions in RAID0, while the remaining parts of the drives are non-RAID. This will give you one 40GB RAID0 partition for Windows, apps, swap and tempfiles, while your remaining partitions (you should be left with two 200GB partitions now) will contain your files and other important data. Of course you can play around with these things further, and allocate additional partitions for different types of files etc. For instance, you can have your downloads, which I assume are disposable, on another pair of RAID0 parititions.
To summarize: RAID0 is best for system and temporary files, where you gain the most speed while having little risk of loosing data. As to your data - always back up your important stuff, and especially anything on RAID0 you care about. If you keep parts of these HDs non-RAID, you can use one HD to backup the other, for instance.
Good luck!