New Gamer

Hello Reddit, Im 15 years old, I'm starting to get into PC gaming. Im starting to play

• Just Cause 2
• Battlefield 3
• GTA IV
• TF2

And some others. However i have a computer (Not Laptop) that my Dad bought for me. (Great computer, Blazingly fast at work) However. I don't seem to get a very good fps on the games I play. Here are the specs on my PC

• Acer aspire x3390
• Intel core i5-2400s CPU @2.50ghz
• 4GB installed memory RAM
• 64-bit

Buying a new pc is out of the question however my birthday is Tuesday and i would really like to spend around 50 pounds making it better for gaming. How could I do so

Sorry for the noob question