Impossible to say since you don't mention what the speed of your current RAM is; only that you have 12GB.
Download and install
CPU-Z. Once installed, run the program and click on the Memory tab. Let us know what it says under DRAM Frequency. If it says ~400MHz, then you MIGHT notices a bit of difference. If it says, ~667 or ~800, then you probably wouldn't notice any difference.
As for your question about purchasing RAM, the answer would be no. The "L" in the list specifies "Low Voltage" DDR3 RAM. However, I'm a bit concerned because not all of the items listed on the Crucial site are low-voltage and UserBenchmark doesn't seem to imply that it cares what voltage RAM you use.
If you can, see if there is a sticker on one side of a RAM module currently installed. If there is, it should list a model number. What is that number?
-Wolf sends