I definitely have to agree with seeing a full system image created before making any large change there. As far as the availability of the Release Preview you can see for yourself that now that 8 is out the option to buy is all you will find at this time. That is any "Genuine MS" downloads would be the retail 8 only!
The decision is yours to make however on which to buy and your actual plans for seeing a dual boot. You wouldn't see it buying an upgrade disk too easily while the OEM disk would be the disk that can easily perform a full clean install of 8 onto a second primary.
That won't cost any $200+ or $300+ like prices seen for previous versions however! You can grab a 64bit 8 Pro disk for $99-. Once you have the 8 media you can also create a new iso to written to a flash drive mage bootable to see the Win 8 install go a little faster then the hour by dvd average time. You can even mount iso images which can speed things up a bit since 8 does take a lot longer then 7 to go on!