If you want to game then invest in a 450w psu or bigger but again you have a cramped computer and when you go big psu and gpu there is more heat in the case and that you don't want.
I thought I saw a good card and it was not very long. I will look for it and come back and let you know if it will work on your system with the 300w psu.
Again you could get a beter system if you have the money to invest. Something to think about.
Okay! That would be great!