Hi Reno
You will not be able to do an Upgrade install from 7048 to the RC builds, you need to do a Clean install and start again. You could use the Windows Easy transfer utility to get your pictures, documents, etc, transfered easily I believe (click start, type windows easy transfer), I believe it will see another partition as a drive it can transfer from, but I haven't done a full test on that yet.
But it can't hurt to try if you want to create a partition for a new build, this way if it doesn't do it you just transfer them manually. During the process it says from a "USB or External Drive", but I think it will do it from a partition also.
There have been numerous stability and other performance changes in the newer builds so I recommend to AT LEAST get the RC 7100 build if you want to continue using W7.