It depends on what you are going to have running on your server, and if you really care about a difference of 1-2 Watts, which is most likely the difference between 1-stick and 2-sticks of RAM (unless you are going for high end RAM that is typically factory overclocked and certified from slower types of RAM and can use quit a lot more power than standard 1.5v 1,333-MHz DDR-3).
If your server is simply going to be a file-server then I doubt it would make the slightest amount of difference going dual-channel, if however you are going to be running a file-server that is also a web-server or used by many clients, or is running any server-side software then you will most likely want to use an extra 1-2 Watts of power by going dual channel to avoid performance bottlenecks.
Anyway, 1-2 W is not worth talking about unless you have also tackled other things that will make a far larger difference to your servers power consumption, e.g. CPU, Motherboard, Chipset, HDD's, and importantly PSU.
For example you could save 1-2 Watts of power by using Wired Gigabit Ethernet rather than N150/N300 Wireless, and that is not a joke, I have seen USB Wifi use 2-W when Idle.
So in summary: Performance wise - it simply depends on what you are using your server for (a home file-server, 1-stick of RAM will be just fine). Power wise - its of less importance than many other choices, but if it is a new build then 1x 4GB stick of RAM is also going to be a little cheaper than 2x 2GB sticks of RAM, and as mentioned you wont need anything more than 1.5v 1,333 MHz DDR-3, although I would suggest buying good quality branded RAM (Kingston, Crucial, Corsair etc that are fully tested and have long warranties).
Could you tell the forum what else your server is going to be made of.? We can make suggestions on parts that will also be low-power, quiet, fast-enough and wont break the bank, most people building home-servers totally over-spec the machine and end up with something that is more expensive, uses more power than needed and is noisier than it should be for its job.
Main PC, P180, CM Silent Pro 500M, Phenom 840 @ 3.6GHz, 8-GB @ 1,866MHz, 256 GB Samsung 830, 500-GB 7K500, MSI 660Ti Twin Frozr, PC is quieter than my monitor :o
Server, 6-TB RAID-5 array, + 2 x 2-TB backup drives, 380W Enermax Pro82+, 4x very quiet fans, positive pressure only, no exhaust fans
Living Room PC, 3500+, 2-GB RAM, HD501LJ