If you client really needs data redundancy, you can consider 3 levels of backups for him:
1. Concurrent backups. RAID 1 would do it for your case, like already mentioned. The benefits are: one hard drive copies (mirrors) the other one almost instantaneously; if one fails, the other one can be used to boot up the OS with all data intact. Disadvantage: if you make a mistake (say downloaded a virus), the mistake will be copied to the 2nd hard drive as well, so you need image backup below, in case you need to go back in time.
2. Scheduled data backups. Like you said you can use just Win7's backup util or others. Do this daily, for only the data and to an external drive. I would recommend to use eSATA drive, since it's a lot faster than USB 2 for large amount of data. Benefits: if the whole PC / hdds failed, you still have all the important data intact (at least up to the last time you back it up), which you can load them up using any OS / PC. Disadvantage: to keep the backup process light weight and fast, you want to only backup the data; because of this you can't restore OS using this method.
3. Image backups. Do this once per week or month, depending how concern your client with his data. You can use image software like Acronis or others. Make an entire image of the OS hard drive. Benefits: you can keep an archive of images (or incremental images), which you can go back to if needed. Disadvantage: could be time consuming to generate an image, and needs to schedule down time (can not use PC at same time) to do it.