Hi,
I know when I tried installing MOSS/WSS on 2k8, I ran into some errors when directly installing, and could not continue.
From what I have read, MOSS SP2 is supported on Server 2008R2. Here is the official SP team blog:
http://sharepoint.microsoft.com/blog/Pages/BlogPost.aspx?PageType=4&ListId={72C1C85B-1D2D-4A4A-90DE-CA74A7808184}&pID=507
With this being said, the ability to slipstream MOSS into the 2k8 build may be the best way to install. There are multiple guides on this subject on the web. Here is one:
http://insomniacgeek.com/how-to-install-moss-on-windows-server-2008-r2/
Before you upgrade, you should create a backup of the site (I prefer the stsadm command for doing this) and at least a backup of the content databases being used.
Another consideration you might make is using Hyper-V, which is included in 2008 R2, to host MOSS in a virtualized 2k3 environment. I don't know your exact requirements, but Hyper-V adds many features that may be beneficial in your case.
Hope this helps.