It's definitely going to be an odd RAID 0. Since you have 120 GB and 256GB disks, you'll see different performance on the first 240 GB of your RAID since it stripes with the 830, but you will be hit by the 320's performance so I'm not sure what your net performance will look like. On the remaining portion of your RAID, you'll be entirely on the 830 with performance to reflect that.
EDIT: I agree with the guy above me. I wrote my response under the assumption that you must have one larger volume, but if you can deal with two smaller ones, definitely don't RAID the drives.
Do not Raid 0 the drives. Keep them separate. You will lose TRIM support and degrade the life of the drives.
It is simply NOT a good idea. This has been asked about once a day lately on this board. Simply don't do it.
Get the maximum benefit out of each drive and keep them separate. If i were you I would use the 256 as the system drive and the 120 for data storage, given the performance of each drive.
Thanks for the help. I already have the OS on 120 gig, i can use the 256 for games and other stuff? I have about 30 gig left on my 120 gig drive. Now i can put the os on the 256gig one since that one is much faster than the intel 120gg.
Depends on the work you want to put in it. In real world scenarios the performnce difference would probably not be worth the effort. If you are happy with the current performance, just use the 256gb as the data drive. Then move stuff over to it from C. You will have plenty of space then on the OS drive.