If you have a stock cooler with the CPU, then you won't get much out of it without possibly doing damage. But ya, if you do have a good aftermarket cooler and can overclock a bit, it should help a bit with 7d2d along with the additional ram. But like SylenThunder mentioned, your GPU isn't too great for this game either... so you really need to consider whether or not it's even worth investing anymore money into this system at all. If you're just interested in buying ram and not expecting miracles, then it's worth it, but otherwise, build a new system.
I also recommend just going with used or refurbished ram. Depending on how many sticks of ram you have in your system already,... so like if you have just 1 stick of 4GB, then try buying an identical stick of ram so you can benefit from dual channel and not have any conflicts. If you have 2 sticks and have 2 more available slots, you can try to find 2 similar ram sticks of identical MHz and latency (I don't think size matters when it's on a separate channel, only the performance) for maximum stability. Otherwise, just swap out what's installed with new sticks and you'll be fine.
Buying a used GPU is a much higher risk due to bitcoin mining, so I'm not sure it's worth it.