You have 2 drives, so only RAID0 and RAID1 are options. Since you want performance increase and max. storage space, RAID0 is your choice.
The improvement is usually significant, but more noticable in large file transfers than in small files. I'd say my guestimate is about 25%-30% increase on average
There is no way to create a RAID array without losing all data. Thus, a backup should be made. I don't know about any websites that are able to store GB's of data. I would recommend to get a DVD burner and burn the data to DVD's. DVD burners are currently in the $40-$50 range, and I expect that you have to pay for a website that can store GB's... It's a good idea to have a backup medium anyways. (not to mention bandwith.. even with fast internet, uploading GB's will take a long time; upload is normally much slower than download).
Final note, realize that RAID0 will increase risk in case of disk failure. However, I like the improved speeds myself.
This website has a little more info on RAID:http://www.acnc.com/04_01_00.html