I think somebody has said that the aperture size should be set to half of your RAM amount, if possible...so in your case 0,5*384 = 192Mb so you could set the aperture size near to that. but I suggest you try to reduce/increase it a bit and see if it affects somehow...aperture size means the amount of memory that your video card can use of your RAM for doing it's stuff...so giving it the whole of your RAM would mean it can eat up all of your memory - and giving it something like 16Mb or less would mean it can't use your RAM too much, which frees it to other processes but on the other hand reduces the "power" of your video card, if it's memory becomes full and it slows it down..if you know what I mean.
try out something like 196Mb (if you can set it to that..I can't remember the values you can give to it)
EDIT: so basically it's just balancing between the memory amounts..system needs memory, but so does video card if you use it a lot. basically I'd say that you only need much memory given to your card if you play 3d-games or such things, that make your card in flames
so if it's not for gaming, you can safely give a smaller amount of aperture size, like 64Mb as you said you had done. it gives your system more memory (so the card cannot eat it all up...altough it might not do that after all, anyway) so it won't get too slow.
