Silverstone now has a few mini ITX Sugo cases. They're all the same thing, except for color and front fascia. These are kind of the de-facto "gaming" ITX cases for the time being (I know of one company coming out with a competitor by Q2 2010). These cases are "limited" to 9" long graphics cards. By "limited" I mean that is the official maximum length. Some people have fit 9.5" long cards, and some have modded the chassis to fit even longer cards. The power supply is 300W and provides a single 6-pin PCIe plug. Basically any card that only requires one 6-pin card and is under 9.5" or so should work. It does fit dual slot cards.
For mobo/CPU, currently the only one widely available is the Zotac supporting socket 775. For CPU, if you can afford the higher price and can find them, go for a low wattage chip. For instance there is the Core 2 Quad 9550, and 9550s. Both same speed but one uses lower voltage.
If you can wait, I would suggest waiting for 3-6 months. Here are the reasons:
In 3 months we might start seeing socket 1156 mini ITX boards. Both Intel and DFI have shown such boards. Socket 1156 uses Core i3/i5/i7 chips. You can have dual and quad cores, with or without HyperThreading. They use DDR3 and have integrated memory controllers, plus socket 1156 has basically the whole Northbridge integrated into the CPU and so the mobo won't need much cooling, plus there will be some power savings (typically chipsets are made on older process than CPUs).
In 6 months we might see a more powerful power supply. A Silverstone representative has reported that they will be making a 450W SFX power supply (the size used in the ITX Sugo) that will support two PCIe power plugs.
I suspect we might start seeing more on the ITX 1156 boards by CES in January, so it is probably worth waiting for unless you need a system quick.