New
#31
My mainboard is an Asus P6T Deluxe (X58 chipset). If its manual is to be believed, it supports both dual and triple channel memory configurations. (I've only used triple channel, though.) Triple channel is to be preferred, but I believ that it's not required.
The Radeon HD4850 ought to give better performance at high resolutions than the 5750, at about the same price. The 4850 has a 256 bit path to its VRAM, while the 5750 has 128 bit. (The 4850 isn't a DX11 card, though.) The 4850 is the least expensive card in the 4800 series, which was the highest-end single GPU line of its generation for ATI. The 5800 series is the top line of the newer generation, and much more expensive. The 5700 family supports DX11, but it's more of a midrange series.
The Socket 1156 approach might be a good one, if you wish to go Intel and you're interested in saving some money. In the US, the I5 750 is about $89 less than the I7 920 (www.newegg.com), and P55 mainboards from the majors (Asus, Gigabyte) can be had for roughly $100 less than the least expensive X58 boards. Socket 1156 only includes dual channel memory support.
If you choose the I7 920, overclocking is common. I normally run mine at 3.8GHz, with a Thermalright Ultra 120 air cooler. I have the older stepping, and I believe that the newer one may do a little better.
The Antec 900 will accept large graphics cards, which makes it a good choice for a mid-tower. (You may have to remove an 80mm internal fan on the back of the drive cages; no great loss.) I still prefer to have the PSU at the top of the case, though; call me old-fashioned. I prefer larger cases like my current one, a Cooler Master Stacker 830, but it's a bit expensive. (Got it for about $175, including shipping. It's only the silver model, though.)