you can always re-partition the drive, and as long as you have at least 2 drives you can implement at least one raid level.(assuming you have a raid controller)
*I'd suggest porting your files to the 400GB drive, set up the raid array and then port the files to the raid array in that order!
as for the rest of your question...it really depends on what you want to do.
(high performance, data security, or a little of both)
There are several different raid levels, each having strengths and weaknesses.
The most common are raid0, raid1 and raid5
most motherboards with built-in raid controllers also support some hybrid levels (as do many raid cards)
examples of hybrid levels: 0+1, 1+0 (yes there is a difference), 5+0, etc...
read up on raid here
, and here
Bottom line: if you want performance/disk failure protection and have a little money, then raid is for you.
if you're like me and don't really worry about disks failing then I'd suggest raid0
if you also want data protection I'd suggest 0+1 (a raid1 array made of 2 raid0 arrays, 4 disk minimum)
if your motherboard doesn't have a raid controller you'll have to buy a raid card
...and I really don't know anything about raid cards...so newegg anyone?
a pci-e x1 card will work in a pci-e x4, x8 or x16 slot as well