dont buy a soundcard if your going to buy pro tools.
it will come with its own interface.
I would recomend Protools over cubase because most big studios will run protools
take into account when your pricing things out that with cubase you will need a sound card and with pro tools you wont.
now for the important stuff.
you must decide now how many inputs you are going to need into your computer.
if you are getting sound from devices that have stereo out then you will only need stereo in. for just vocals and turntables for samling.
if you are wanting anything more than that you will need more ins. a fully equipt MPC will have 8 outputs for example. so you will want a sound interface with 8 ins. or else you will have all your samples jammed into stereo. if you think you might want to record drums then you will want to have more inputs.
I have a soundcard that has stereo in m-audio audiophille 24/96 which rocks and is very cheap. but i now record some beats with a live band and im screwed. so keep that in mind.
and if your buying a mixing board you will want to run that fictional MPC thru your mixer with 8 ins and 8 effects sends and 8 outs. or you might not need all that.
best thing to do is decide what exactly you want to have, and (very important) what you will want to fufill your needs in any future gear purchaces you migt make along the way.
then go talk to the guy at your local music store. you live in st louis so theres gotta be tons. go to at least 5 if you can find them. get all the ideas. bring paper, take notes. then come back to your internet and read reviews and specs of all the gear you found out info on.
then Draw your studio on paper. not the layout of your room but the wiring diagram. may sound complicated but your going to school for that stuff so im assuming your apt enough to do that. most in music production are usually a little smarter then the rest.
with your drawing you can see that you wont run into any input output difficulties, you also will be able to create a list of the cabling you will need as well.
remember your sound is only as good as your weakest link.
go here
http://www.long-mcquade.com/index.asp?section=5&level2=5&level3=9&id=38 and configure your pc properly.
buy good cables
back to what you want. just vocals? then you will only need stereo in, a good mic and pre amp.
you have a great budget for that, seeing that you already have your PC.
i bet after those classes you are going to take you will want more so dont limit yourself.