well nfl2k4 uses a peer 2 peer xbox live type system, while nfl2k5 from what i know uses a server-based (theirs) type system.
and from all reports from the field (of pathetic losers) -- is that the nfl2k5 hosted servers are very unreliable and slow. The nfl2k4 online experience thus was much faster and smoother cuz of the different online backbone system they used.