Well, I don't know myself how it works, but you have to find a solution to free as much memory video as possible.

So until Michael comes here, maybe you should try :

1) reduce your desktop resolution (640*480 or 800*600) ... it's always few mega free
2) disable your second moniteur if you have dual screen
3) close every other softwares, explorer windows etc, ie windows etc ...
4) reduce MoI window as much a possible
5) If you're under vista, maybe disable Aero and all the little tools (I'm not sure how much they impact on the video memory)

256 Mb should be enough for a 6000*6000 ... but maybe it requires more memory than I think.