If you have 2 250gig drives in a raid 0, you still only have 250Gb of space. The data is striped on both drives and when accessed, reads off of both at the same time.
Why don't you save the files to the drive you want them on in the first place?
No, if you stripe your data across two drives, you end up with a total volume that is double the size of the smaller of the two drives. In this case, you'd get a 500 GB volume.
There are other ways of combining multiple disks into a single volume rather than using RAID. These involve using specific file systems that may not be compatible with Windows. It may be the case that if you want to use Windows, you'll have to go with RAID. Let me dig into the topic a little and I'll see what I can find.
Edit: I think that what I'm thinking of is just a software equivalent of JBOD. With a spanned file system volume, you still have the chance of major data corruption if one drive fails, so it's not necessarily safer than RAID 0, and it dosn't provide you with the performance benefits.