I already told you that your computer is a piece of garbage and you'll never be able to game with it. A slimline computer with a 220W power supply is for computer illiterate people who want to check email and play farmville. Get it through your head quickly please!
I did have a friend that jammed a full size GPU into a slimline. (according to spec sheet it supports pci-e x16) Of course he couldn't put the side panel back on and he had to keep it on it's side to keep the card from breaking. And he had to keep the PSU on the outside. This should would be the best you can do for your system. I don't play LoL so I couldn't tell you what to expect except that it should be quite a bit better then what you have now.