It depends on what you are doing...
I have applications that store very large amounts of data in VB6 in memory at one time...and very rarely have out of memory issues.
Where there are problems is when loading controls with data (such as tree views, list boxes, etc). There are certain memory constraints on the amount of memory that can fit into some controls. In the old days, (VB4), there were controls that could not store more than 64K of data in them. I am not sure how many more of those types of controls still exist.
I'm not sure that is your problem here...but I haven't noticed any memory-specific issues between 98 and XP using VB6.
Hope that helps.