All of these systems are completely pointless. All you need is s decently specced PC, smallish form factor if you're worried, just anything with an HDMI output (which all video cards have now), and some USB ports.
Hook it to the TV, plug in a wired 360 pad, switch Steam to Big Picture mode... done!
I agree with this however don't go for small form PC's, upgrading them is a pain in neck.
The two things which will make a difference to gaming are memory and graphics cards.
Memory is a pain to upgrade at a later date as demand gets higher than supply whenever a new tier is released. For example DDR-3 RAM is now cheap as chips but DDR-2 is expensive, as manufacturers are keeping with the times. you can't use DDR-3 RAM in a DDR-2 motherboard so your forced into paying higher prices. Therefore it's advisable to get as much RAM as you can, even if current games don't fully utilize it yet.
Graphics cards are the other way round. As the form factor remains pretty constant you can upgrade easily in 3-4 years with little hassle. Unless you plan on your machine being seriously beefy, I'd pick up a mid-range card for £130 now, and then upgrade it in 3-4 years time for another £130. Logic being is that there isn't much difference between a £130 card and a £260 in the grand scheme of things. In 3-4 years, your £260 will be almost as ancient as the £130 one, but a new £130 card will be using current technology so you've spread the cost.
The only other thing to mention is SSD's. If you are serious about gaming then they will speed loading times up no end. Still a bit expensive for most people though.