Images are kept according to your userid, so when you upload, it all goes to the same folder.
Image names are not stored intact. Everything beside "a-z", "0-1", "." and "_" is deleted from the name.
And finally to answer your question =) no, I don't think images are overwritten (they weren't in RC2). They are suffixed with a 4 letter numerical string.
Let me explain it better.
If your userid is 318, your images are uploaded to "gallery/files/3/1/8/file.ext"
If you image is called "nice flower #3.jpg" it will be renamed to "niceflower3.jpg"
If you upload a new image called "niceflower3.jpg", and already exists a "gallery/files/3/1/8/niceflower3.jpg", the new image will be renamed "niceflower3_7864.jpg" where 7864 is a random number.
When processing, if the filename already exists, it tells me "Invalid File Type". If you look in the FTP folder, the filenames have all had the random number appended to the end of their names. I go back to the upload screen, check the box to process the files in the FTP folder a second time, and it works now that the names are different.