Cindy, you shouldn't need to blow away SQL Server 2008.  You should be able to install SP1 on your current instance, then install CU3 for SP1.  At worst case you will miss out on a couple of fixes that were in 1812 but not in 2723, so if you are actively relying on one of those fixes, you'll want to verify whether it is not in 2723 at all or if it was inserted in an earlier CU for SP1.  That should be small if anything, and those fixes will usually return in the next CU.

Cindy, you can't apply 2723 unless you first apply SP1, so there is no case where you can "skip" SP1 to get a later build.  However you should note that the builds are nowhere near equivalent.  In many cases a fix is introduced in one branch or the other (RTM vs. SP1) and only later (or sometimes never) ported to the other branch.  These updates are no exception.  I have updated the main body to show the 71 fixes that are shared between these two builds, and how most of them only seem to be in one build or the other (most seem to have targeted SP1; and those that are missing from there might already be there from a previous SP1 CU).