What do you use your HTPC for? If its just Kodi, then I would suggest looking into OpenELEC . I'm not familiar with the hardware of the Revo RL80, so you may need to check their site for compatibility, but, I'm running it on my old Zotac ION1 box and it boots in like 10 seconds.
OpenELEC is basically a stripped down "set-top" implementation of linux that boots directly into Kodi. I use to run variations of Ubuntu on it, but Openelec is so much faster. If you're only using your media center for Kodi, then this is your best bet.
Try openELEC.
The easiest install method is to have 2 USB sticks, one that you make into an install stick (A) and another one (B) that you install onto from A. That way you don't need to mess around with your hard drives.
It's basically as barebones an XBMC install you can get, with a super lightweight Linux foundation. If your hardware still stutters playing back hi-res stuff, theres not much else OS-wise you can do, since openELEC is super slim and includes graphics acceleration.
It's also super easy to use, since it's pretty much just XBMC.
I agree go with roadglider505, Zotac ION 1.8ghz Atom from Newegg. Best bang for the buck but it needs ram and something to boot. So I am thinking about the OPENLEC XBMC system running off a usb drive or hard drive. It will do the NVIDIA ION and also 1080p on the HDMI. OPENLEC Plus save the money that you would have payed for Win7 by going with the XBMC OPENLEC, plus it auto updates itself. I am looking into doing this myself. From their website.
What is OpenELEC?
"Open Embedded Linux Entertainment Center, or OpenELEC for short, is a small Linux distribution built from scratch as a platform to turn your computer into a complete XBMC media center. OpenELEC is designed to make your system boot as fast as possible and the install is so easy that anyone can turn a blank PC into a media machine in less than 15 minutes."
yeah, I partitioned it and loaded xbmc live, so no interface. I am thinking of loading openElec. it's a precofigured "slim" linux version that takes up minimal resources. it's supposed to make the revo more like an appliance.
You can read about it here.
For XBMC you can install any linux operating system (fedora/ubuntu) and just download XBMC on top of that. Otherwise OpenElec is an XBMC derivative that should work for you: http://www.openelec.tv/get-openelec
That is an advantage of the ION based HTPs, they can play virtually anything under the sun, however they are significantly more expensive than an ATV2 (think ~200-250 bucks). I personally have the Jetway nettop, with openelec on a 16GB (could go much smaller if I wanted) SSD.
Given the space constraints of your 500GB network drive, re-encoding HD videos into H264 isn't necessarily a bad idea, as you can only hold so much on 500 gigs. The ATV2 should be able to handle playing any kind of SD video.
For HTPC under Linux there is no better way than XBMC imho. I never really tested PVR with it though. If you are looking for a straight forward - easy to use XBMC install go and check http://www.openelec.tv/
Try out unRAID for your storage solution. I have been using it for about 6 months and love it.
For my media center I just bought an Acer Revo 3700. From my research it should be able to stream 1080p just fine.
I intend to use OpenELEC.