The 4.6.3 to 5.0.0+ upgrade is pretty bad, so give yourself some time and test it out thoroughly. Since you're already on Postgres 7.3, you're lucky -- part of the reason the upgrade is so bad is most people were on 7.2.
I think 7.4.6 is fine, but I haven't tried it myself yet. I'm on 7.4.5 I believe, and that is working fine.
