I've got a Revo 3600 and two Revo 3700s - they're great little lozenge-shaped boxes for use as a media centre. I wouldn't recommend them as your main PC though, but the specs for the successor to the Revo 3700 is pretty good.
That successor is the Revo RL70, which Ebuyer.com have been selling for a while now and is currently a stonking £171.08 with free delivery, though stocks will go fast at that price. It's using an AMD E450 with HD 6320 integrated graphics, both of which are different to the CPU/GPU in my Revos so I can't comment on whether that's better or worse really.
You get 4GB RAM, the awful Linpus Linux (replace this with Ubuntu+XBMC+tvheadend) and a good-sized 640GB HDD (rumour has it that some people have received 750GB HDDs when ordering the 640GB version!), so the specs are spot on for HD media centre use.
Considering there's an offer on for a Raspberry Pi for just £29.95 including free delivery until 20th August - put WEBFREE in the comment box when buying to get the free postage - I've been ruminating whether to buy one or not. A colleague has and he says it's good, plus I already have all the extra bits you have to get (SD card, USB phone charger, HDMI cable, USB keyboard/mouse, ethernet cable). As a replacement for the Revo for XBMC duties, I remain less convinced about the Pi because it doesn't have hardware acceleration for MPEG 2 (and presumably not for Divx/Xvid either) and it's only recently that's I've been downloading MPEG 4 content (which the Pi does have acceleration for).
I think that as for playback of 720p MPEG4 content, the Pi is a good choice, but I'm quite surprised that Farnell sell loads of accessories (including an overkill of a 23" monitor for £138 - surely Pi buyers will already have an HDMI-equipped display?), but not the most desperately needed one: a bleeding case! It's the one major thing I don't like about the Pi - a case is considered an optional extra or even not sold at all. There's no way I would want to run a computer with the entire circuitry open to the elements and because of this, I'll wait for a model with a decent case (and maybe MPEG2 licensed...or a better CPU and/or GPU to decode it properly).