For a computer to send and receive MIDI events, it needs to have a MIDI device. This device can either be a dedicated MIDI board, such as a true MPU-401 MIDI board, or a soundcard with MIDI support, such as my Creative SBLive. Which ever you choose, you'll want to make sure the MIDI device is supported by the Linux kernel you are using. Dig into the documentation directory of the kernel sources. It's a great source of information about which devices are supported by the included drivers. MIDI devices have been around long enough that support is almost assured.
Once your MIDI device is recognized by the kernel, you should have access to it through a /dev/midi device file. Your sequencing software will use this device file to communicate with your computer's MIDI ports and ultimately your synthesizers.