I just followed the mac mini route, this is my experience so far:
I bought a the lowest end mac mini, the $599 i5 model from amazon. The CPU is way more horsepower than you'd need for typical HTPC applications, but I had to immediately change out the 5400rpm stock HDD to a 256gb SSD because the stock HDD is really slow. I got the Samsung 840 pro SSD for $230, but cheaper ssds exist. This isn't a must, but it really makes the system snappy overall. This is especially noticeable when you launch apps or index large files (such as accessing a large movie collection on my DS412+ nas... there's no lag, its all instant, in all my tests with other computers running HDDs, there was always a bit of a lag scrubbing mkv video files but not with this ssd) I'm also going to upgrade the ram from 4gigs to 8 ($70). So just on the mac mini alone, you're looking at around $900.
Why'd I go with the mac mini?
right now, its the best built, quietest, lowest power consuming powerful PC out there, period. Nothing comes close. You turn it on, and without the SSD you dont even know that it's on, there's a fan but you dont hear it unless you literally put your ear to the machine or put your hand in the back to feel the hot air blowing out.
The box itself is gorgeous... all aluminum, and has a tiny footprint. For pure aesthetics, you wont find a nicer package, even if it's almost twice as expensive as other boxes.
Anyway, I've installed windows ultimate on it, via bootcamp. Apple makes it super easy to install windows, however, if you go this route, make sure you use a usb 2.0 flash drive and NOT a usb 3.0 drive. Took me 6 hours to figure out that a usb 3.0 flash drive wont boot the windows installer.
I'm waiting on the HD homerun people to release their 4-tuner DLNA complaint box this summer, but with windows installed, it should have no problems integrating into my setup. Right now my mac mini boots directly into windows media center, and I have the plex client installed. There's a shortcut in windows media center to jump into Plex when I want to watch my archived movie/tv show library. I went with plex over xbmc because plex is much easier to get set up, i think, than xbmc is. XBMC just has too many options, that frankly, is a bit overkill for a normal user. Plus, I never could get xbmc to correctly recognize the metadata in my movies/tv shows folder on my nas. With Plex, that was never an issue an getting metadata was much faster.
Finally, my avr equipment is pre-hdmi. So I needed a computer with both optical out and hdmi out. Insofar, only the mac mini has this compatibility out of the box. It was super easy to get this set up in my system and the drivers worked immediately.
There's also a way to set the system up to work solely with the apple tv remote, which I have done. The only limitation to it is that you get 7 buttons of functionality as what's on the apple tv remote. Those 7 buttons control plex easily and fully, as well as windows media center, so far, but I havent setup the live tv portion just yet so I cant comment if I'll eventually have to get another mce remote. I'm hoping I wont and continue with the clean setup that I presently have.
It's no doubt an expensive way to go, but if you are looking for the ultimate, clean setup for a set top box that does it all and has high aesthetics, you cant beat the mac mini route.
another option for you may be to get an Intel NUC:
it also has a tiny form factor, low heat, and a fast enough cpu to get the job done (if you're using it just for htpc). You will have to buy a SSD, ram, wireless network card and ir/remote to get this to work, and come in for about $550 or so.