Yes, you will have the same power consumption of the drive. As for the speed of WD Green, it has Intellipower which means that if your application requires mostly sequential read/writes, then the drive will perform comparable to a 7,200 RPM drive. However, if your application performs mostly random mode operations, then the performance may drop by about 10% because of the latency time.
However, if you are planning to be leaving your system up all the time or are considering moving to a raid based system in the future, then you can take a look at WD Red, which are designed for such environment and might suit better for you. Here's a link if you want to check them out:
http://products.wdc.com/support/kb.ashx?id=b6Hnfe
Hope this helps.
Cheers! :)