I guess it's what you were concerned about that would help figuring out what you need to get.
If I remember, the Infinite Crisis Secret Files & Origins
main story connected the dots in such a way that you wouldn't need to buy the separate mini-series (or, I guess their TPBs). It explains how Alex and Superboy-Prime set up everything to lead into Infinite Crisis
As others said, I would recommend The OMAC Project
and Villains United
and their strengths as stories in and of themselves. Furthermore, I'd recommend Checkmate
#'s 1-25 (the follow-up series to The OMAC Project
), as well as the current Secret Six
series (and I guess the mini-series that came out shortly after Infinite Crisis
, though I didn't that that mini was as strong as the currrent series).