You should look into throttlestop first.
https://rog.asus.com/forum/showthread.php?2329-ATTENTION-G73-53SW-Owners-CPU-Throttling-to-798mhz-an...As for memory your laptop uses ddr3 10600s -1600 memory.
I have a G74sx laying around to repair and it has Samsung M471B5273DH0-CH9 memory sticks. If you are going to upgrade it's best to keep everything the same/similar.
You can find them on ebay here
http://www.ebay.com/itm/121619285257?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITIf you have 2 of these currently installed (8gb) you could buy another 2 for a total of 16gb. It will cost about $50 and as Clint suggested might make a slight difference or none at all.
What I suggest you do is upgrade to an ssd. The hard drive that comes with the g74sx is usually only 5400rpm. The specs page i pulled up for the bbk9 edition specified it shipped with the 1tb hdd. If you have a 5400rpm 1tb hdd for your operating system I can only imagine how slow the laptop is.
Here is a list of sdd's that will work with your G74sx
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&N=100008118%20600038519I would suggest to stay away from refurbished ssd's, I've heard nothing but horror stories about them.
After you buy the ssd you will have to reload windows, the asus applications, and the drivers to make all your hardware work correctly. You will also have to install the critical updates from windows.
After all this is done the ssd will change your boot time to about 15 seconds or so. You will also be able to use your current hdd as a secondary drive for data. If you need the adapter (some g74's didn't ship with them) you can buy them used on ebay.
Keep in mind that the G74 has 2 motherboard versions. One has the 3gb gpu, and one has the 2gb gpu. Your laptop has the 2gb gpu since it's the best buy edition. If you see graphics settings for games posted on forums, you need to verify that they are for your laptop or you won't achieve the same results.
You might also want to clean the heatsinks/fans and repaste since the laptop is older. You can monitor your temps with hwinfo64.
It all depends on how far you want to go.
Let us know if you need more info or have any other questions.