Well, the last motherboard link you posted doesn't work, but generally if a motherboard works with a higher speed, like 2000, it will also work with a lower speed, like 1600. Read the reviews for the mobo and see if you can confirm this (look for people who mention the speed they run their memory at) as I am not 100% sure of this and I'd hate for you to waste money. But when I was building my new system about a month ago, I looked at tons of motherboards, and every one I can remember looking at seemed to have a "max speed", and supported all speeds below that as well. Sometimes manufacturers leave stuff out on the specifications. You could also ask the manufacturer if the board worked with it for some assurance; it really makes no sense for a board to work with every DDR3 speed EXCEPT 1600, especially since 1600 is probably the best-value memory right now and there's a lot more demand for it than any other speed besides 1333.
For your needs, I would not spend more than $250 on a motherboard. Most mobos that cost more than that are geared towards gaming, with PCI-E x16/x16/x8. For your needs since it seems unlikely you'll be interested in SLI or CrossFire, you just need two x16 slots- the third slot can be x8, x4, or x1, since you probably won't need it for anything.
Mobo selection to me is such a tedious process. There are so many to look at, that all seem about the same! A lesson you can take from that, though, is just narrow it down to mobos that suit your needs without going over the top in price, then pick the one with the best reviews. That's pretty much how I do it, at least; not sure if you're comfortable with this sort of strategy or not, but it has worked out great for me.
Keeping with the roughly $250 price limit, that narrows it down to the GA-EX58-UD4P, P6T SE, and GA-EX58-UD3R. The specifications on the GA-EX58-UD3R seemed to be incomplete; I could not find where it listed its expansion slots. Because of this I cannot comment much on it. However the remaining two are both very good boards, and while I've never owned a Gigabyte product, I can recommend Asus for their good quality, value, and tech support. The UD4P costs more primarily because its PCI-E slots are x16/x16/x8/x4/x1, if I understand correctly. That's a lot of PCI-E slots (more than you need, and faster than you can make use of, most likely). For this reason I think the P6T might be the better choice due to its lower price, however it's worth noting that the Gigabyte board does give you great value for all these PCI-E slots. Also I can guarantee the P6T's compatibility with 1600 memory (I have a P6T Deluxe V2 myself, which doesn't seem to be very different from this P6T, and am running my memory at 1600), while as I mentioned in the first paragraph I am not entirely sure about the Gigabyte board.
As for your memory, the XMS3 4GB you have in your list is dual-channel; I'd stick with triple-channel as it is not much more expensive and is much faster. The other Corsair sticks are only 1333MHz; I'd go for 1600. This leaves just the OCZ memory as the only option I'd recommend from your list. Corsair is the best brand in memory, though; you may want to consider this
RAM. It is slightly cheaper than the OCZ RAM (and also has free shipping), is made by Corsair, and has 18 reviews with 100% 5-star ratings. The only downside is that its latency is 8, while the OCZ memory's is 7. However I doubt you will notice this performance difference. Also, the OCZ memory states in its title that it is "Low Voltage," which would be great for overclocking. Unfortunately this is not really the case as 1.65V is both the standard and the maximum safe voltage for RAM on a LGA1366 motherboard (the Corsair memory is 1.65V as well).
As for a CPU cooler and thermal paste, if you are purchasing a Retail processor (which you are- OEM processors are the ones where the picture is of the actual chip instead of the box), a cooler will be included with thermal paste already applied. I am using the stock heatsink on my Core i7 920 which is slightly OC'd to 2.68GHz. For major overclocking, you may want to purchase an aftermarket heatsink, but for basic needs the stock one works just fine. Most people do not buy thermal paste if they're going to use the stock heatsink either since they come with it already correctly applied, although aftermarket heatsinks do NOT have this pre-applied paste, so you'll want to buy some if you're going with one of those. It's hard to tell which thermal compounds are the best, but I know that the ones that contain a lot of elemental silver are higher quality. My advice is to save some money here and not buy either of these things for the moment, though.
As for a video card, I am not familiar with the one you've got, however this webpage
is tremendously useful for deciding whether investing in a new one is right for you. While most of the information there is too technical for me, the FLOPS (floating-point operations per second), core clock, and memory type make for good comparison points- look for GDDR4 or GDDR5 memory and high values in FLOPS and core clock. If the values aren't too much higher than what you've got, you may just want to stick with it.
If money runs tight I'd recommend hanging on to your current card and seeing how you like it. You can always upgrade the video card later, and if you won't be doing any gaming or 3D editing there's not much reason to drop the dough for a new card unless you're just wayyyy behind the times and need it to support higher resolutions or dual monitors or something like that.
Hope this helps and good luck with your build!