You can't upgrade / modify the graphics capability of a laptop,
AFAIK. You are stuck with whatever is provided.
For serious gaming, a laptop just isn't going to have the VGA grunt (and HEAT) that a dedicated, high-end card in a desktop will give you.
I don't know enough about media centre & TV-out to give an opinion on interfaces available, but you don't need that fast a CPU to run a media centre PC, so you will certainly find a laptop CPU fast enough.
There's not a great deal of point in worrying about the size of hard disk - you can easily connect an external USB disk (OK, it's another thing to carry).
Every laptop I've seen recently has built in wifi and a LAN interface, so the network side is easily covered.
You should still be able to buy with XP installed from the big manfrs - until the end of this year, I think - but I don't know about WMC edition.
I have yet to see anything positive about Vista that makes a compelling case for it - quite the contrary, in fact!
I've just made a backup backup copy of my XP installation CD, with SP2 slipstreamed.
SD