Think of it this way...
It takes about 2 minutes to move 128 MB of music on a USB 1.1 connection, that's about 4 hours of music in 128 Kbps MP3 format in 2 minutes. Every second, a little over half a MB is transferred (equivalent to 12 Mbps.) This rate may differ if you have a lot of USB devices on it though, but a mouse and keyboard wouldn't effect it noticably.
So in short you have bandwith to spare if your only going to play mp3's off of it. Video will need to be transferred to an internal drive unless you get a USB 2.0 card.
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