Basically, to get a basic 5.1 surround sound system you'd need 2 front L/R speakers, 1 center channel, 2 rear L/R speakers, and a subwoofer. You could get the entire line of speakers from Diva except the subwoofer since to my knowledge they dont' make subs yet. You still haven't stated if your budget includes the display, and the HTPC either. However, a reasonable setup within your budget would consist of the Diva 4.1($699) front L/R, Diva C3($499) center channel and the Diva 2.1($399) as surrounds. That'll leave you with about $3400 for your sub, receiver, HTPC and display. If the HTPC and display aren't included in your budget, you could stretch for the Diva 6.1($1299) as the mains.
For subs, many swear by the
SVS subs, but if you're looking something cheaper, try the
Hsu Research VTF2 or the upcoming VTF3
There are few reviews of the Diva by Swans. The more notable ones are on
Home Theater HiFi and
Stereo Times. You might need to search for them as they might not be on the main page. There aren't many direct professional comparisons, but many have compared them to the B&W CDM-NT series, the Paradigm Reference Studio series, and some to the PSB Stratus series. Since I don't own any of the above-mentioned speakers other than the PSBs, I can't speak for their performance.
While Woody might be right about the audio part, a HTPC doesn't really perform that bad if you configure it right. Ease of use might be lacking, but a Pronto and IRman/Girder would cure that pretty well. With a good soundcard and S/PDIF passthrough, audio quality is pretty good. What you need to do then is to cut down on the noise of the fans so that all that SNR doesn't go to waste. The good thing about a HTPC is that its easily upgradeable, extremely configurable, and provides scaling options that only a multi-thousand external dedicated scaler can match. The bad thing, as Woody mentioned, is the audio part. Its a comprimise, and the final decision depends on you.