yea definitely best getting a receiver first and building a system from there however for a decent one its gona cost quite a bit more than your budget. When you say 5.1 speaker bundle do you mean like essentially a dvd player with a built in amp which your speakers plug into? I ask because as some times the speakers can be of average quality so might need to upgrade them.
With a new amp/receiver you could run your ps3 hdmi to your tv and then optical to your amp, your pc either optical or coax to your amp and component video to your tv.
I wouldnt bother with a new sound card unless your mobo is quite old and doesnt have SPDIF out, either optical or co-ax. I have a X-fi fatality pro series sound card which I output to my home theatre system and what a complete waste of money it has been, sound quality over the standard HD onboard audio is hardly note worthy, it has some cool features software wise but hardly worth the price tag.