I don't believe anyone sells a PCI card that just supports MIDI. Some PCI audio interface cards have a set of midi ports, but you already have a good audio interface card.
There are only a few manufacturers of midi interfaces given many music products now transmit midi over USB connections.
Your best (and possible only) solution would be to get one of the USB-based midi interfaces. If you don't have any available USB ports (either USB 1.1 or USB 2.0), you would then need to buy a PCI USB card or PCI "Combo" card (has both USB and Firewire ports).
Finally, are you sure you even need a midi port interface? What will you do with it? Most keyboard controllers these days have built-in midi support via a USB connection. Do you have a number of devices that can only be controlled via MIDI?
Thanks guys, I had heard that a pci type midi interface was a bit faster than usb but I already have one of these midi to usb chords laying around.
Nope! MIDI transmission over midi cables (DIN5) is slooooooow compared to modern speeds, at a breathtaking 31.25 kilobits/sec. USB and PCI will be able to keep up and process it just fine. I suppose you could argue the PCI is a bus closer to the CPU than USB, but I don't think you will encounter any timing issues at all.
It'll be interesting to see what the new midi spec brings, if they ever roll one out.