GTX560 Ti in HP 500-164?

I was given a HP 500-164 prebuilt desktop as a gift a year or so ago, and it is currently good enough for me to run TF2 native resolution at highest settings (minus AA) very well. I know that is no real accomplishment, but has been good enough for me being mostly a console gamer. I recently bought a Wii U, and am going to use my PC as my other main gaming device, since I don't want to buy one of each new console, and I want to put a bit of money into upgrading it.
It comes with an integrated graphical processing reading of around 768MB taken from the CPU, I believe, so almost anything would be an upgrade- but I want something I don't need to upgrade for a few years, but not something I will have to spend more than around $100 on. I recently found a very good deal on a GeForce GTX560 Ti, and I am very ready to jump on it, but I want to know it will work first. I already upgraded my PSU because the original got destroyed (unrelated), and went with a Corsair CX600, so I hope that will be good enough.

I have read that the 560 Ti takes 550 watts, so will that leave me enough power to add a second 8GB stick of RAM? I know electrical theory and mechanics well, but I'm not 100% savvy on computers, so will that work; will I have to buy a slightly less intensive GPU to leave room for the RAM?

Thanks in advance. Any info you people can give me would be so appreciated. I really don't want to spend this money on something, just to have to spend more to make it work.