Get your Revo 3610 from
Ballicom for £160 and, if you need an N rated wifi card, then eBay will sort you out for less than £10 if you don't mind waiting a couple of weeks or so. Don't pay more for a Windows version unless you absolutely must have Windows, as Linux actually performs better in playing H.264 HD videos.
As for XBMC, I am also running this with the Aeon skin on top of Ubuntu as an application that I can exit and return to the desktop. There is also MythTV, which is a media centre written from the ground up for Linux. It has already been merged with Ubuntu into its own distribution Mythbuntu, which makes installation (if not setup) a breeze.
I got Snow Leopard to boot on the Revo for the hell of it and in 64 bit mode, which is more than my MBP has managed thus far, but there are too many bugs with it to prefer it to Linux at the moment. Wanted to try out Plex, as I am more familiar with that, but I think it will take a lot of work and hacking to get OSX to where Linux and Windows are on the Revo now.
I have a 32bit Ubuntu installation for now but am looking to dual boot with a 64 bit setup as this cuts the load on the cpu when playing HD videos from about 40-50% right down to 10%.
I also have the WDTV 1G, and to be honest, it is more than good enough as a media player except it cannot deal with DTS sound (not a big deal tbh) and that it stays very warm whilst plugged in even when switched off. Don't like that. It is absolutely silent though, which makes it perfect for quiet films.
I do prefer the flexibility of the Revo and it makes for a nice cheapish toy. I would try to nab one of the dual-core Linux R3610s before they run out and the price jumps, when only Windows is offered. I had no need of mine as I have plenty of PCs and Macs around the place to play with, but I don't regret getting it for one moment.