9.4 upgrade SHMVIRTSIZE problem

Upgrade from 9.21 to 9.40 Windows NT Advanced server SP3 4 Gig ram Had had SHMVIRTSIZE set to 655360 Did the upgrade and Informix failed on setting up the Shared memory. Worked OK if it was reduced - to 327680, and stepped up OK to 557056, but after that it just falls over when you try to bring the bugger online. Anyone got any ideas? Non Move to *ix realted, thanks. Just don't understand how with nothing but the db version changed that suddenly starts falling over. Even just ideas about what to look at would be most welcome ta Andrew