Not sure how much help I'll be, but saw this thread and might have something to offer...
I bought a couple of Acer Revo 1600's a couple of years back. These were the original Intel Atom / NVIDIA ION nettops. At the time they came with Windows XP, 1GB of RAM, 160GB HD, gigabit ethernet, and a keyboard and mouse, all for $200. This box probably had the biggest bang for the buck, especially considering the inclusion of the OS. After much experimentation with XBMC under XP and Linux, I later decided to upgrade the OS to Windows 7 and also increased the RAM to 2GB. I've been using this quite happily as an XBMC streamer, and am now pretty much converted over to using Plex (due to its easy central-metadata storage on my main server in another room and its support for transcoding to my iOS devices).
The room that has this Revo (my living room / home theater) also has an XBox 360 that I use as an extender for live TV / DVR. My main server is an HP tower with Intel i7 that I got as a refurb online (under $600, if I remember correctly). It lives in my bedroom but is surprisingly quiet (my wife is super-sensitive to these things and has never complained). I have an HDHomeRun Prime 3-tuner (cablecard) box, and a 2nd one still in the box that I haven't gotten around to hooking up. I have Comcast and no premium channels, so I'm fortunate that all my channels are copy-freely, which opens up some other options (more on that in a bit).
At one point I did experiment with WMC7 on the Revo, and just yesterday I actually dusted it off for that purpose to see if I could get it working acceptably. My mindset recently has been that I'd love to have a single UI for everything, or perhaps even just a single hardware device that might need to jump around between apps, so the idea of using WMC combined with Plex and some other WMC or Plex plug-ins/channels for Netflix, Hulu+, and whatever else, got me into the tinkering mode again yesterday.
Well, I don't know if I might have crudded up the display drivers at some point in the past with various codec packs (back when XBMC was still going through some growing pains), but I found the WMC experience to be really, really bad. The WMC GUI was sluggish compared to my XBox, but I could possibly live with that. What wasn't acceptable was that the picture would stutter a lot. I've read some of the same things you have online, with other people reporting success, so there's a part of me that's contemplating doing a fresh Win 7 reinstall just to see if some old video driver garbage might be the culprit. But I also recently bought a refurb 2012 base-model Mac Mini and I'm probably going to make that my new living room box instead. If I do the Revo Win 7 reinstall, I'll let you know what happens. Who knows, maybe you'll be interested in buying mine.

A couple of other ideas...If all you really want/need is a 2-tuner DVR that's under $500, you might want to consider buying someone's used TiVo HD with Lifetime service. I think you can find those for under $300 on eBay. In fact, I used to use TiVo and got an email offer to get mine re-activated with Lifetime service for just $50, but I was out of town and missed the deal. I may still call them to see if I can swing it, if for no other reason than to sell it on eBay or use it in my daughter's room (she has an XBox 360 as well, but I can't get ethernet to her room easily so she clogs up my WiFi bandwidth when she wants to watch TV on that - fortunately she's been more interested in watching Netflix and Hulu+ on her Apple TV (gen 2) lately). Again, if I *do* get that TiVo deal, maybe you'll be interested in buying mine.

Or, you could get one of the newer 2-tuner TiVo Premiere models for $100 plus $15/month for the service. Lower up-front cost and it "just plain works", but more expensive longer-term than building your own PC. TiVo Lifetime service costs a hefty $500.
So the other thing I played around with yesterday was the Netflix and Hulu services on my HP Tower (i7 CPU) in my bedroom. I have an Apple TV (gen 2) in that room, too, and that's been the preferred device for Netflix and Hulu, but as I mentioned, I kind of wish I could have one single device for everything and not have to change TV inputs. Well, Netflix seemed pretty good and there's already an icon for it in the WMC menu. Video quality looked fine and the remote operated things as I would expect/want it to. Hulu was a mess, though. I installed Hulu Desktop and tried it out. First try: I selected an episode of Revenge (a show my wife is into and was in our "Favorites" list). The Hulu app ended up going into what seemed to be a non-stop loop of commercials, and then finally started playing...the Jimmy Fallon show. At that point it also wasn't responding to my remote. I had to get up and kill the app and restart it. Next try worked, but I still was having remote troubles. Finally closed it down altogether. I also had to use a different hack I found online to get it to add an icon for Hulu in the WMC menu, but that didn't properly set the focus back on WMC when I exited out of Hulu. As for Amazon Instant Video: I don't think they have a standalone app that can happily live alongside WMC. I think you have to launch it from a web browser. That doesn't sound wife-friendly. Moral of the story: My experience has been that these online services work much better on other consumer electronics-oriented boxes (the ATV experience is excellent).
Do you already have an XBox 360? If you can get an ethernet cable to it from wherever you might already have (or be willing to store) a desktop computer, you could also go that route. Going that route, you might not mind as much if the desktop PC is a bit bigger and louder. The XBox 360 "just plain works" as a Windows Media Center extender. The newer "slim" models are much quieter than the old ones, but they're still louder than they should be. There's a fan inside the power brick, but the power cord is long enough that you might be able to hide the power brick behind something to quiet it down further. I believe there's also a fan inside the XBox 360 itself, but that one seems to be nearly silent.
Quite honestly, if all you really need is a 2-tuner DVR, the used TiVo HD w/Lifetime service might be something you should seriously consider.