You should be able to hit 3.0 pretty easily with relatively little extra heat on the stock cooler.
Definitely not worth it. I know you only have 4 slots, but you can run triple channel on your board with 3x2Gb (leaving slot 2 empty). I would go with 6Gb first and see how that works for you. You're honestly not going to see much difference. If you do decide to go 8Gb (sorry...screwed up the link)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231330
Otherwise, go with a 3x2Gb kit of the Dominator or XMS3, or whatever you choose with good reviews.
The thing with voltage is this - power usage is lower (going from 1.65v to 1.35v dropped my power usage by about 20w - not huge, but noticeable). However, it also allows you to bump up the voltage (say, to 1.5v) to get a little overclocking or better timings. Overall, though - voltage isn't that big of a deal. Just remember to keep it 1.65v or lower for i7s.
I don't over clock memory because I've never had success with it so...I stopped trying. The problems I continue to have with 90% of the PCs I build always seem to be memory related. To me, they seem to be the most fragile item in a PC (2nd being the power supplies) so I simply don't risk it if I don't have to. The other issue to keep in mind is memory blue-screens due to incorrect timings, a failing stick, bad voltage, etc... have a MUCH higher risk of screwing up a Windows install than anything else. I can't tell you how many times I've had a memory stick go bad and have totally hosed a Windows installation. It was more common with XP and I haven't seen it nearly as much with Vista/7, but I have still seen it. Most of the time, it's not a concern, but there's nothing like seeing the "windows found a corrupt registry and had to restore it...the restore was successful" or something like that, followed on the next reboot by a "hive" error and no way to get into Windows. I'll OC a processor until it bleeds, but I've never been a big fan of memory overclocking. That's just me, though. I'll bet for every one of me, there's 9 others that say it's harmless. I'm probably in the minority. LOL.
Bottom line - go with 6Gb of 8Gb (if you're okay with spending the extra $50), but not 12 or 16Gb as I simply don't think you need it nor would you notice it. 3x4Gb sticks (12Gb) are almost $600 for the cheapest set, while 4x4Gb are over $700. That's insane with a great set of 4x2Gb sticks is only $250 and you'll simply never notice the difference between 8Gb and 12Gb. I'd recommend starting with 3x2Gb and seeing what you think and saving the $50 first, but if you went with 8Gb I wouldn't blame you. Heck, I went 12Gb and think that's asinine.