My father's desktop is in terrible shape, but has tons of room for improvement. I am planning on putting in a Geforce 550Ti 2GB on Friday, and the desktop has 4GB Ram, and 1TB hard drive, but the CPU is the problem. The CPU in it is a "Intel Pentium CPU G630 @ 2.70GHz"..This is very bad. if I go ahead and put in a new GPU, will I be able to max this game, as well as Battlefield 3? It's not the card I am worried about, it's the CPU.
Depends on which CPU you get. Some of the large multiplayer maps for BF3 like quad core CPUs. So any of the Core i5 2xxx CPUs would be a good choice. However, that 550Ti is what will hold back BF3's performance the most. It's okay for medium settings, but anything higher than that requires a considerably stronger graphics card.
It is your graphics card that will hold you back. a GTX550Ti will get you low settings, maybe, depending on your resolution. Just set it to AUTO and see what it gives you. I previously had a GTX580 and couldn't max the graphic settings at 1920x1080. My GTX680 does max the game and I am willing to bet a GTX670 will as well. However, the game does scale very well and even at medium settings looks really good. I would put what you can into a video card. Also, what PSU do you have? You need to make sure it will handle your card.
PS, your dad's PC is not in terrible shape. It's a basic modern PC. At least you are working with a modern CPU.