I've been out of town this week, and just got back. I'm going to try to update my original post this weekend with the details for setting up MPC-HC to launch as an external player, and am hoping to also include the details necessary to launch it via a .bat file that will mount .iso files, so that you can play DVD and Blu-ray .iso files.
I wanted to comment on a couple of things. I feel like I'm sounding like a broken record on this, but I really think that the $200 Revo has the biggest bang for the buck. The Windows XP install it comes with is lightweight and boots fast. XBMC for Windows coupled with MPC-HC plays Blu-ray 1080p rips great, and with 5.1 audio (or stereo, depending on your needs) over HDMI. And this is with the lowly 1GB of RAM that it comes with. This only costs $200 (plus another $20-25 for a USB IR receiver/remote).
If Windows 7 or added memory or the dual-core CPU brings something important to the table (as an HTPC / media streamer), then fine, but I'm not really seeing any proof of that. So why spend $330 (+ USB IR/remote) when you can get away with spending $200 (+ USB IR/remote)?
XBMC Live didn't play my test video as smoothly as MPC-HC under Windows and I couldn't get 5.1 audio over HDMI working. So, again, why would you want to go to the extra trouble of installing another OS when the stock OS works as good or better? The only thing XBMC Live/Linux offers is that the built-in player is ION-optimized. Is that a big deal? Well, it's the ideal way of doing it, and it allows for the on-screen status bar, etc. to be displayed which is nice, but it's not worth the added trouble, IMO, if it means that I need to pay for extra RAM (and deal with voiding the warranty to install it) in order to get smooth playback, or have to wait for 5.1 audio to be fixed, etc.
So again, sounding like a broken record here...just buy the $200 model, keep Windows XP, and go through a few one-time steps to get it all configured right, and be happy!