The USB on keyboards is for MIDI signals. If you have a USB then it means that you really don't need a MIDI interface, you can connect your keyboard to the PC via USB and install proper drivers to write MIDI tracks in a software sequencer, control a softsynth, sampler etc ...

If you want to record audio out from your keyboard then you would need an audio card. USB or PCI? well, this depends on what you are trying to achieve. Are you planning to record just the keyboard or do you plan on recording more instruments and do mutli track recording? (Like recording guitars, keyboards, drums, vocals ...)

If you plan on recording only keyboards with may be vocals, then I think USB 2.0 is just fine. It's easy to install and it would work great for a few tracks of audio. If you need more inputs and outputs for multi track recording then go with a PCI card, USB 2.0 doesn't seem to handle multiple tracks of audio very well also there are latency issues with playing back audio in a multi track environment.

I have an Echo Gina for multi track recording, MIDIMAN MIDI interface for my audio workstation for multi tracking and an Ego Systems USB audio interface for my laptop for Soft Synths.