1) Drives can have different sizes, caches, and speeds. But to some degree you'll be wasting capacity and performance of the faster/larger drive.
2) Putting an existing single drive into a RAID 0 set will wipe all the data on it. You'll need to back up any files you want to keep to another disk (or DVD or whatever) so that you can restore them once the new set has been created.
3) Windows 7 includes the RAID drivers for most of the motherboard RAID chipsets. Since you'll have to reinstall Windows anyway after configuring your RAID array, you should get the required drivers automatically, so there really isn't anything to worry about.
Don't forget to have a backup strategy in place. If you don't have one already then that alone would make it worth getting two new HDDs so that you could use the third, older one in an external enclosure for backup.