But have you read and followed any of the pinned posts and howtos according to update to a newer rom version? As far as I remember (had been a while since I flashed my i9000 from eclair, you need to root first, then flash cwm, then minimum an ics kernel and then the rom. Mayb the cwm step can be skipped, because recovery is also coming with the rom as far as I remember. Flashing via odin a kernel should be ok. Then flash a ics or jb rom of cm first and then an update to RR shouldn't be a problem.
But again, read read and read again, and as soon as you are sure what you are doing, flash....there are couple of posts here and in the i9000 section too.