The short answer is that there is no standard. for instance, feature films can range from several GB each to 300mb, depending on compression, quality, etc.
Again, it depends on how it is encoded, it's length, quality...
Media file sizes aren't a standard.
I just put an 8GB card in my Bold. Prior to that, I had a 4GB card with over 200 songs and 3, 1-hour TV episodes, and some Word and Excel files. The songs can be at least 3 minutes...some are club extended mixes nearing 8 to 10 minutes. My Office files were a few kB to almost a MB.
The best thing you really need to do is just try it out and see.
If you take an average of 3.5Mb per song (3 minute song encoded at 128Kbps in mp3 format) then you can at least make a guess. As others have said, your mileage may vary and the only way to know for sure is to load the card up with music and watch how the free space goes down.
Bear in mind also that you will only see 7.6Gb of space once the card is formatted in the phone. This is because the OS uses some space for caching and stuff.
You should therefore get about 2000 songs on an 8Gb card.
I keep my musci orgaized in several folders and this seems to speed up access as the phone does not have to wade through 000s of filenames to find the one song I want. To see how much space is used by any given folder I use Tree Size Free from Welcome to JAM Software and check the individual folder sizes whilst the phone is connected to my PC in Mass Media Mode.
