It appears you are on a budget. Well, who isn't nowadays?

1. Good mobo.

2. Good CPU, but do you mean 2 of them? Any chance of upgrading these to a higher clockspeed, like E5540 or higher?

3. Not good. With DDR3 you need at least 3 sticks to benefit from the DDR3 architecture. I would get 12 GB (6x2GB).

4. Nice disk for media. Not so nice for a boot disk. I would suggest a Velociraptor for boot (OS & pragrams) and at least 3 of these 640 G disks or larger for storage. Look at my guide about storage for setting up your disks.

5. Video card is OK,  although I'm personally more in favor of ATI (4870/4890).

6. Forget about a SB sound card. Use on-board sound. SB has crippled too many systems to be advised in any system.

7. Good choice.

8. I would opt for Win7 Professional 64 RC at the moment and upgrade after 10-22-09 with the official release.

9, 10, & 11. Case is a very personal choice. Fans may be necessary with the mobo and CPU of your choice, but in general Intel stock fans are not the best. PSU may not be the best choice. I'm not familiar with the brand and model, 750 W seems sufficient, but I suggest you also look at more familiar brand names. Have a look at some of the links I provided in my guides to see what are good PSU's in the various tests.

General question: Why go for a very good mobo with good CPU's but skimp on memory and hard disk setup?

I suggest you reread the PC buying guide and Storage guide, because you may well be better off with a highly OC-able i7 than with dual Xeons.

How to get the best from a PC? Some guides...

I just checked the mobo info you supplied the link to and indeed it appears to have no on-board sound. If you stick with this mobo, you have to get an audio card, but then also check the slot layout on that mobo. Can you use a PCI-X card or do you need a PCI card? You only have one PCI-e slot, so you will not have the possibility of adding a PCI-e raid controller. Then there is no sense in getting a SLI capable PSU, since the mobo does not allow SLI. If you need an audio card, I would have a look at Turtle Beach instead of SB. Overall the mobo seems to be lacking PCI slots. You better consider what expansion capabilities you want or need in the future, so you will not end up in a situation where you just miss the extra slots needed.

Mostly personal preference, but also influenced by their clockwork accuracy in bringing out new drivers each month and the fact that there have been some problems reported with nVida cards lately.

Absolutely, since it allows the Xeon 55xx series and these are much faster than the 54xx. If budget allows it, you could go up to dual W5580's and it has on-board audio, so you don't need that SB card.

However, it misses firewire connections, so my preference would be the X8DA3. This mobo has a better placement of the PCI-e slots as well as more memory slots, PCI slots, plus two firewire connections and on-board audio. Look here: http://supermicro.com/products/motherboard/QPI/5500/X8DA3.cfm

Both video cards are good. Take the one where you get the best deal.

The memory is sold in kits. He needs two kits of 3 x 2 GB to get to 12 GB total.