The RAM doesn't really "Buffer" the data that is to be written to the disk. However your applications work to load data from the hard drive into memory, when you have data that is to be written to the disk that is done nearly immediately, depending on how busy the system is. Buffered writes are only very short term.
The faster your hard disk is, the quicker it will complete the operations it performs. So, for a given read/write operation, a faster hard drive will be "active" for less time than a slower drive. So while it may consume more power, it does so for a shorter amount of time. Overall power consumption may be more, less, or the same depending on your usage habits.
In my experience, once you have "enough" RAM, having more RAM will not help hard drive performance. My current computer is quad-core with 8GB RAM. Usually RAM usage peaks at about 4GB. It rarely goes above that. If it has been running for a few days without rebooting, it may get to 4.5GB or slightly more. Only if a program is "misbehaving" (preparing to crash) will RAM usage increase significantly above that. Of course, that's only "my" experience. It could be that the applications that I run are just not demanding enough on memory to take advantage of the full amount of RAM.
Even copying 200 GB from one physical drive to another will not noticeably increase RAM usage. It is purely dependent on drive speed.
I think overall, you shouldn't sacrifice speed for power, especially on a computer with only 1 or 2 drives. As time goes on, more and more applications are more demanding on RAM and disk activity. Eventually you will wish you had purchased the faster drive. If you have many computers with many drives then of course, power and drive cost may be a more significant factor.
If you are concerned about noise and power consumption, you could look into some of the "green" drives. I have no experience with them, but I believe some will normally run at 5400 RPM and will increase their RPM as demand increases (probably up to 7200 RPM). That could be a good balance between power, noise, and performance. I have been planning to try one of these the next time I need a new drive.