Will getting a DAC converter improve mp3 playback

I am thinking of getting a DAC converter as I mainly use my computer for music. I have a collection of mp3 and wma (192 kb/s and 128 kb/s) and want to get most out of it. I was also thinking of getting the USB thingy from blue circle (http://www.goodsound.com/equipment/blue_circle_usb_thingee.htm) and then connecting my computer into a receiver using an optical out. I am a college student so money is tight, I hope to spend no more than $300 on whatever I do. What do you suggest. Which will sound better?