Win98 will give "out of memory" errors if you go to 768 MB. THis can be fixed by editing certain system files.
I wouldn't bother with 512 MB of ram in win98, unless you need it for photoshop or video editing (in which case you should be in win2k or xp
). You aren't going to notice any difference in AH or most games, that's for sure. Windows itself will not be any faster. (I've heard WW2ONline is a ram pig and will eat up 512 MB though, so if you play that game it might help.)