Better yet, don't waste your money on shipping and get something from Craigslist or a local thrift or used record/CD store. I picked up a pair of Polk Monitor 7 speakers for $40 and a Caver TFM-15CB amp (the third I've got) for $80 from Flat Black and Circular, a popular music store in East Lansing.
The Polks were a ridiculous steal at that price - but apparently nobody even knows what they are anymore, even at a vintage record shop. You'd probably have to pay around $500 for a new pair of speakers to equal them.
I've heard stories of people finding Klipschorns (or was it some other Heritage Klipsch?) in thrift stores, McIntoshes free by the side of the road, etc. You're probably not going to find anything like that, but there's lots of deals out there all the time. One thing to keep an eye out for is Klipsch Heresys - they're all over the place used because pretty much every church and auditorium used them at one point.
Of course, all of those are really too big for what you want - desktop speakers. I don't have much experience with monitors, but I do think you should look out for vintage models. I'm a big fan of vintage Infinities - the EMIT tweeter is superb. I wish I had a pair of Infinity Infinitesimal (the originals with a Watkins woofer), Modulus, or Kappa 5 bookshelf speakers. All of those are somewhat rare but usually go for $400 or less (except the Infinitesimals in very good condition).
I've always wanted to try a pair of Boston Acoustic A40 bookshelf speakers, but I've never had a chance. They're a lot cheaper, but with a good amp will probably perform as well or better than any of those computer monitors for a fraction of the price.
You'll need a decent amp to go with any of them, and unlike some here I agree that the DAC makes enough of a difference. If you really can't tell the difference between a PC sound card's combined headphone jack & line out, you ought to upgrade the rest of your system or get your hearing checked.
I'm a big skeptic with audio, as an engineer. Unfortunately, I don't have the means of setting up a meaningful blind test (no dB meter and rarely anyone willing to assist), let alone a rigorous double blind test. I do think that many things make a big enough difference to be obvious without a blind test - like switching between most speakers, an under-powered amp, or with a poor performing source.
I had high hopes when hooking up my uDAC to my stereo as a source. I had been pretty impressed with the uDAC so far as a DAC/headphone amp, but I wanted to see how it performed on my main system. I have a pair of Infinity Renaissance 90s, powered by an Adcom GFA-555 on bottom and a Carver TFM-15CB on top, fed through a Carver C-11 preamp and with an NAD 5325 CD player with upgraded caps as a source. The R90s are absolutely amazing - read the reviews at audioreview.com if you doubt me. They don't have a perfect 5-star rating on that site with 33 reviews for no reason. As a whole, the entire system is dynamic yet extremely neutral - it gives almost no distinct sound signature to reproduced sound.
Anyway, the NAD 5325 was NAD's first budget CD player - it's from the early nineties if memory serves me correct. I bought it used with the upgraded caps, so I have no idea how it sounded stock. As is, I've never noticed any flaws in it as a source - even through the Infinities. I've often wondered if upgrading to a modern CD player would make a noticeable or even drastic difference, but as long as I'm able to keep it running I doubt I'll get a newer/better one.
So I figured that hey, the uDAC is a modern DAC - and at $100 and 20 years newer it ought to perform very well - if not handily exceed the performance of the NAD.
Well, I hooked it up to my preamp, using my Thinkpad as the transport. Well, I was disappointed to say the least. Compared to the NAD, it's quite muddy in the bass and lacks the same level of detail in the highs. There is much less separation in the soundstage as well. You can go ahead and call it the placebo effect if you'd like. I don't really care. There was definitely a big difference in how the two sounded. And it's not what I expected.
Now, once you get past the point of neutral sounding DACs, I do seriously question the level of improvement. It's the same thing with cables, I think. Get speaker cables as short as possible with low enough resistance and there's no difference past that. The difference between 20 year old, 30' long 16 Ga wire and brand new 10' long 12 Ga wire when driving a pair of 4 ohm speakers is quite noticeable.
Anyway, I hope you find something you like. If you have the space and the time to find a pair of vintage bookshelf speakers and an amp to power them (I recommend sub-100 Watt amps from Carver, NAD, or Adcom to start with). An external USB DAC would help but is definitely less important than the speakers.- the transducer that actually produces the sound is by far the most influential component in the system. Spending an extra $100 now on the speakers/amp is much better than spending the money on a cheap DAC and wishing you had bought better speakers.
-Ben