Considering that a whole brand new laptop or desktop PC can be purchased for around $400 and will probably easily out perform your six year old PC, perhaps the time has come to move on to a new machine? A new graphics card might help with performance in your six year old machine, but don't expect miracles. If you are still running Windows 7, then there is little that has changed over the years other than more bloated software and a move towards higher definition video. You certainly appear to have enough RAM with 16Gb on the motherboard. Hard disk size is adequate!
Have a search around on Google to see if others with your system set up have experienced the same problem and whether they have found a solution. PPRuNe is a good place to ask for advice, but the real computer nerds tend to hang out elsewhere.
Cleaning the dust out of a PC while you have the case off is always a good idea. It may not help with speed, but it does make the system cooling run more efficiently. Reseating cards is just an industry standard trick to get things going again. You would not expect to see any visible corrosion or oxidation on the connectors. It just forms as an invisible microscopic layer. Unplugging and plugging in the connectors is sufficient to scrape through any oxidation layers and restore a good connection.
At a guess, I would say your system will benefit from one of the lower end gaming cards if you are interested in watching video. Something around the $100 mark or less should work and boost the graphics performance considerably from 2%.
Make sure the card will fit the connection socket in your computer and that the ourput connections on the back are compatible with your current display. Adapters are available, but they can be somewhat bulky, expensive, and provide an additional point of failure.