How much RAM to purchase?

IDE possible try to get crucial 16gb kit. It just flies even with many heavy applications running on the laptop.
The virtual disk usage is literally 0 with 16gb at disposal.

There's a lot of reasons your software could be getting slower, but, using your virtual memory could certainly be one of them.

If you're going to upgrade the RAM, 16GB is, in my opinion, the way to go. It's not very expensive, in the end. I got the Crucial 16GB kit for under $80.

Another fantastic upgrade is a Solid State Drive (SSD). This will really improve performance of all aspects of your system, especially working with large files (provided they are ON the SSD)

Totally agree and I second that. I bought an SSD as well and with 16GB ram it has changed the character of my MacBook pro.
It's unbelievably fast now and it boots in less than 20seconds and apps as heavy as light room open in less than 35seconds.

If you are looking at SSDs keep crucial as your last choice. They are very good in memory but not so good in SSDs. Try samsung 830 instead of 840(new version) since the new version is not good. The best one of course is the Vertex4 SSD which is the fastest of all in today's market.