I would suggest getting a Wyred4Soound DAC-2. Right now there are 2 on eBay for $300-400. This unit also serves as a pre-amplifier so that is one less piece of equipment you have to buy. Then start by feeding it from your optical out on your motherboard. I know that the conventional wisdom is that a computer is a noisy environment that degrades sound but when Stereophile reviewed a PC sound card they found no evidence of this. If you might consider a dedicated amplifier in the future this would be a good way to go. There are lots of options in the used stereo amp market. If you are ever thinking of Magnepan speakers I would suggest a Parasound HCA-1000 as it supplies the high current that Maggies need. $200-400 on eBay.
However if you want to go with a better receiver then you could get a reasonably good DAC from Schiit Audio (or others) for $99 and that would get you into hi-rez pretty cheaply. This does limit your upgrade path to a better receiver.
BTW, you can sometimes find really good deals on used audio equipment on eBay, Audiogon or US Audio Mart.
A word of caution. It is possible that anyone's pursuit of better sound can lead to a lifelong quest that will suck time, energy and money out of your life.