All right, I've successfully filled up the cache to 4GB using a LargeAddressAware FF8_EN.exe. At cache_size=700 I had no trouble, so I upped it to 800 (manually in the VS debugger), still no trouble, then to 900 and it crashed when the memory overflowed. I think 700 will probably be a good size for 64-bit users to max out their RAM usage while avoiding a crash. Obviously, you have to have at least 4GB of RAM to do this. And if you're wondering why my initial guess for a proper cache_size was so far off (2000+

) your guess is as good as mine, but I'm thinking I must have dropped a zero when I first did the math. Replacement textures are generally 1024x1024 pixels and each pixel is 4 bytes for a total of 1024x1024x4 = 4MB per image, plus a little bit of overhead for the cache logic. For 2GB of RAM that comes out to 512 images, but of course we have to allow for FF8's memory usage which is why we drop to 250. For the additional 1-2GB enabled by LargeAddressAware, we can safely add 200 and 500 images to the maximum cache size since the additional RAM will not be used by FF8 itself.
Still, please be aware that I don't think this large of a cache is necessary. You should be able to enjoy Tonberry at smooth speeds with a cache_size less than 250--at a size of 700 we're replacing textures the game loaded 20 minutes earlier. Many of these old textures in the cache are going to be backgrounds and character textures from an area you've long since departed and the speed gains from keeping these textures in RAM are going to be minimal.
Last thing, this is a known issue I will be working on: if the cache has been full and you've been playing for some time without seeing any text (dialog, menus, etc), the cache will dispose of the high resolution font textures and you will start to see text with mixed high-res/low-res characters. This is because, unintuitively, the process that loads the textures is called very rarely for the most common textures, and if they're not actually seen in game they will fall to the back of the cache and eventually be removed. I will be working on a way to keep the most essential textures in the cache no matter what.