If you are planning on using 710 (latest stable release), please note that MythTV recordings need to be stored on the system drive. Other than that, it also depends on how many MDs you use, as the Core will have a complete image for each one.
I agree with nite_man, the Revo looks very interesting. Only downside I found is the lack of SPDIF, but if we're able to get audio over HDMI to work, I'm a happy camper. Didn't find any info on the network chipset, but presume it will work when 810 is released.
Edit: What the hell, checked out the specs at Play, and they state that Ethernet LAN is only 10/100...?