Welcome to Bleeping Computer.
The maximum amount of RAM that you can install in this computer is 32GB, I don't see any problem with adding another 8GB to make 16GB of RAM. If you want to add more RAM I would suggest purchasing modules which will be matched with the existing module/s. To find out the make and model of the module please do the following.
Please download and install Speccy to provide us with information about your computer. Clicking on this link will automatically initiate the download.
When Speccy opens Click on File which is in the upper left portion of the screen, then click on Publish Snapshot.
In the following screen click on Copy to Clipboard.
In your next post right click inside the Reply to Topic box, then click on Paste. This will load a link to the Speccy log.
As for a graphics card, how much do you have budgeted for this. The specifications for this computer show a 450W PSU, but it doesn't show the make and model of the PSU. There are companies that still try to snow people by stating that the PSU has a "peak power rating" of (whatever). This is not what the PSU is capable of producing on a regular basis. So the question is what is the actual wattage and what is the amperage rated for each of the rail voltages? This is important because if you were going to use a GeForce GTX 680 you would need a PSU capable of producing at least 450 Watts. You now can see that there is a relationship between the GPU and the PSU needed to support it.
Edited by dc3, 23 July 2017 - 12:37 PM.