Firstly I don't think there is a way to take a full backup of your current installation so that you can revert back to it. It is a bit like upgrading from Windows Vista to Windows 7. If you don't have an original Windows Vista installation disc you are not going to be able to revert back to it once you have overwritten you hard drive with Windows 7. The best thing you can do is download the closest 2.1 stock ROM to yours from www.samfirmware.com
. The closest one I can see is I5800XXJG6.
Secondly make sure you have installed the latest version of Samsung Kies (so you have the most up to date drivers).
Unzip the apollo_0531.zip. This will create a folder in which you will find the apollo_0531.OPS file.
Unrar the I5800XXJPB.rar file. This will create a folder in which you will find the I5800XXJPB tar archive file. This is flashed as 'one package' in ODIN.
I don't know what Odin_with_OPS.zip and I5800XXJG3_XXJG7_OXXJG5.rar are so ignore them.
Now right click on I5800_Odin_Multi_Downloader_v4.252.exe and Run as Administrator.
Once ODIN has started follow the instruction in the original post.