360kbps mp3?
assuming you only have onboard audio (on the motherboard)?
assuming you have decent headphones?
as far as what is necessary and what is optional that depends quite a bit on how decent your onboard is and how picky you are. as long as your onboard does not have issues such as buzzing, crackling, distortion, etc you might be able to get away with using it if it powers your headphones fine. if you think about it like this than soundcards/dac are optional. however, there is certainly more quality to audio by using such products but whether or not you care or need such things depends on many factors.
soundcard or dac+amp units? depends on your preferences and needs. a soundcard is a good choice if you need to hook up an analog non usb mic, analog speakers or need virtual surround capability. a dac+amp unit is nice if you want a physical volume knob, better sound quality for the money (they focus on only 2.0 sound not extra features) and something that is portable.
decent soundcards? the dx has the same dac as the $200 stx but doesnt have the strong amp. decent choice if you arent using hard to power headphones. the z has a strong amp but an average dac. decent dacamp? the e10k is a decent budget product while the d1 is a good combo unit for a bit more money.