PaterAlf: I don't think you'll miss much when you skip
Villains United and
Day of Vengeance, but if you can pick them up for $5 it's probably worth the money.
At that price it's a steal. VU is usefull to know just what is up with Luthor andother villains. It's not strictly necessary, but it provides good context, and it's a gret mini series, leading up to a equally good, if not better, Secret Six series right after the Crisis.
PaterAlf: Another interesting book might be
JLA: Crisis of Conscience (JLA #115-119).
It's a pretty good prologue to the IC. Nothing there that actually needs to be known, but it's still good to have it, if only to see where exactly everyone stands at the start of the IC, how the League broke down. Since the theme of all the build up and IC itself was how the heroes screwed up, how the bonds between them broke and the central question of IC is not whether they can beat the bad guy, but if they can fix themselves I think it's good to read through all of that build up to really get into that situation, feel everything falling at seams, not on some cosmic level, but on a very personal level, just like the character are seeing it.