Alternative to PST that works like PST?

Outlook XP. could upgrade to 03 for the espanded PST limit but…

Is there an alternative the PST that works and looks like a PST. The users can create folders in Outlook that are oustide of the actual mail store but show up in outlook folder view just as a PST would. Something with a more stable back end that could hold GB’s.

Anything out there?>