Not really. If your existing hard drive is failing, you need to get the data off it now, not mess about with RAID. The only thing RAID would do now is copy the disk (in RAID1) and it will be much slower than just cloning it directly.
Options are:
1. Clone the existing disk to the new one. New drive then becomes the master drive and system boots off it.
2. Get new drive, install it as master, reload the operating system. Then install old drive as a slave and just copy user data off it. (Easier if you have a dedicated partition for user data, otherwise data is splashed around the windows system
3. Get new drive and mount it in a USB caddy. Copy teh user data and as much configuration data as I could off the old drive. RMA it, wait for teh replacement then re-install operating system, copy over user data and configuration files from the external caddy.
4. If you don't have a lot of user data and you have a DVD writer, write critical and user files to teh DVD - saves buying another drive.
5. Use Windows backup (if you installed it) to backup to a series of DVDs - but if you didn't install it, don't waste time doing it now if the disk is on its way out.
6. Get new drive, reconfigure the system as a RAID array (which may compromise the data on the existing disk anyway, depending on how the RAID controller works) then wait for ages until the new disk has copied the data over, and hope it doesn't fail in the meantime.
Personally I would go for option1 using a Linux live CD to do the cloning - or if I was really worried about the data, option 2 or 3 and woryy about the OS later, once my data was safe.
Hope that helps