Grabbit: I also have 768 RAM. Always got that message with Win98. Now have Home XP. Runs beautifully. Nothing dangerous anymore..
As I believe someone alluded to earlier, W98 (and I believe Wme) uses a fairly primative memory manager which is little better than the old W3.1 days to maintain backward compatibility with vendor software that the vendor's never bothered to update. Aside from the obvious limitation that the fixed resource page introduces (i.e., getting a message that you're running out of memory when you actually have half a gig free because you're actually running out of available resources), the primative memory manager also allows resource handling bugs (whole lot of them out there in commercial software) in applications to eventually starves your system and a practical limit on how much system memory the OS can actually make effective use of. If you're loading more than about 128-256MB of RAM on a machine or trying to do more commercial/industrial things (i.e., you're using your computer to do something serious like daytrading rather than just playing Duke Nukem and managing your home finances with Quicken), you really should look at upgrading to W2K or Wxp rather than trying to use W98/Wme. Good luck.
Experiencing the same thing with Win98SE. Upgrading from 256 to 512. I tried one stick of 512, didn't work, thought it was the memory, so I exchanged it for 2 sticks of 256. Still doesn't work. My motherboard is an Asus A7V. It can support 3 sticks of 512. The extended POST memory tests fine. And a DocMemory test in DOS tests fine too. (www.simmtester.com) So it seems the memory is compatible with my motherboard, but it's not compatible with Win98SE. I try to run Norton Diagnostics and do the memory test, and then I get "Error detected at address 147005440" after a few seconds. That address changes depending on what's loaded. If SAFE MODE I got the error again, but it said "Error detected at address 234397696". It seems the error I get is the border of what is actually free. Safemode loads less programs, so more memory would be free, hence the high address. Not sure though, just a guess. When I load a few programs, eventually it will say I'm low on memory. Sometimes the system will freeze or crash. I will try some of the previous suggestions and see if any help. Skram