Kingston and G.Skill both have 4Gb ddr3 dimms that run at 1.6v or less. G.Skill even has a 16Gb kit (Ripjaws Series), it's not cheap, but it's out there! Be careful though most 4Gb ddr3 dimms (and higher) are ECC only (which this board does not support), but Kingston and G.Skill have some that are not, including the kit i just mentioned. There maybe other brands, but you'll have to research that. Be sure the check the supported memory list for this board. Lots of Intel boards are very picky about ram.
I did a quick check of supported memory and the only tested so far has been the Kingston brand. Personally I would stick to that list if I were going to use such high density ram and drop that kind of dime. As for where, just do a search (Google Shopping, Pricegrabber, etc.) for the brand and type you settle on. Companies change prices and stock all the time, so I could recommend one and the other might have it cheaper. Best to do a fresh search and compare pricing. Good luck!
4GB dimms are not price competitive, so get some 2GB dimms.
When the price of 4GB dimms become reasonable upgrade.
only buy dimms that adear to JEDEC, for ddr3 thats PC3-10600 1.5V
issues i had:
Memory manufactures make statements that their products will work with one system or another, but in the real world, often they do not. As I stated before, best policy is to check the boards supported memory directly http://www.intel.com/support/motherboards/desktop/dp55kg/sb/CS-030685.htm
So far, Computer Memory Test Labs (3rd party tester for this board) has only tested Kingston memory. In the list that was tested, they do have two options for 4Gb dimms, one at 1066 and the other at 1333. As 8steve8 said, $400.00 per dimm is not very cost effective. Though if you really need to have 16Gb total, then it looks like the Kingston modules would be your best bet.
All that being said, we all don't follow the supported list, especially one this short. Personally I'm using Mushkin memory (EM3-10666). I've used Mushkin for several Intel builds over the years and have had zero problems, other than the occasional dead stick, but they have an excellent warranty program. Problem is Mushkin has yet to produce 4Gb ddr3 dimms, so the max memory you could run on this board will be limited to 8Gb. Long story short, maybe the Corsair or the G.skill will work, maybe it won't. Until more memory is test with this board you just won't know for sure. Good luck!