"OK, how I can found out if the keyboard will connect using USB and not MIDI interface?
Let's say Motif ES or Pa1xPro?
"

New Yorker, both of these keyboards can be connected via MIDI or USB to do MIDI work. For USB connection, all you would need is a USB cable, for MIDI connection you will need a MIDI interface on your computer.

Again, keep in mind this is only for MIDI signals, you cannot transfer audio from your keyboard using the built-in USB connector into your computer, for audio you will need an audio interface (it could be a USB, Firewire or a PCI interface, but it is a seperate interface than what's already in your keyboard).

You may want to tell us what exactly you are trying to achieve with a sound card:

Do you need a Pro or consumer level card?
Are you planning to do multi track recording?
How many audio ins/outs do you need for your work?
Do you have multiple instruments that you need to connect to your computer via MIDI?

All these questions depend on whether you should go USB, Firewire or PCI route. I see responses with pro level to very basic consumer level recommendations and IMHO you need to let us know what you are trying to do first.