2.0 to 2.1 volts Is what the package says. 4-4-4-12 timings
So you will want to make sure the BIOS sets this value correctly. Just go into CMOS setup and set vdimm to 2.0 or 2.1. The smart thing to do is to put on 1 stick of ram only on first boot, go into bios and make sure all the RAM settings are good, and then add another stick, or, since you will be installing Vista you may want to install Vista on 1 stick only and then add the second stick, in case you have the 'above 3 gig' problem.
Also, to get your motherboard to recognize 4 gig you are likely going to have to go into the BIOS and enable 'memory remapping', on my Asus board that is what I had to do.
OEM Vista should work fine, OEM is supposed to be tied to one PC only and not transfer to another later on, in case you build a new system or heavily upgrade this one. Full retail lets you install the OS repeatedly. I believe MS is fairly forgiving about this though. At some point, say if you put in a new motherboard, you may have to call them and just tell them your MOBO died to get permission. You get no tech support from MS on OEM but you probably don't care about that.
I'm in the same boat as you - helping a friend buy a brand new system. Mine is over a year old (e6600 @3.5 ghz, 6 gig ram, asus p5b deluxe.) So I too am trying to determine a good mobo to buy. We finally settled on the Asus Maximus Formula
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131227
and a similar Gskill 4 gig kit you are getting. So we can compare notes lalter. Just PM me if you need any help and if you learn anything cool let me know.
Good move waiting for the 45 nm Penryn chips. My friend was anxious to get going so he just got the Q6600, maybe upgrade later. This Asus board will handle Penryn, I'm pretty sure. Penryn has more cache and a few improvements which will amount to a significant improvement, clock for clock over existing chips but the big thing may well be the OC ability. Some test chips hitting about 4.5 on air cooling.
Only problem is the 45 nm Quads are delayed and it sounded like March would be the earliest we see them. The duals are coming soon though I think.