No, not too risky. HD don't fail very often - but with two you are more likely to have a failure than with one - but the risk is still small. Still the key is you don't store anything on them. If one fails, which is unlikely, you simply replace it and reload your OS and applications. Naturally you still want to backup all your data on the other drive, but should be no need to backup the RAID drives if only have OS and applications you can reload. Just make sure the mobo has a good RAID controller, which most do in the range you should be considering for a new i7 build.
Are you sure two RAPTORS will be that much cheaper than one SSD?
By the way - when you use two 74GB Raptors in RAID0, your OS and applications are split between the two drives - so you have 148GB of capacity to use for OS and applications - one advantage of going with RAID. Also have you compared RAID Raptors to a single Velociraptor?