Why is my new computer lagging so badly?

Whenever I play games, even games like Minecraft with all the graphics options set as low as possible, I still find everything lagging to an extreme. I can't work out why? I have attached a detailed copy of the Windows Experience Index Report. Please help me, I am so stuck:(

Added (1). Component Details Subscore Base score
Processor AMD FX(tm)-8320 Eight-Core Processor 7.7
Memory (RAM) 16.0 GB 7.9
Graphics ASUS EAH5450 Series 3.6
Gaming graphics 8438 MB Total available graphics memory 5.8
Primary hard disk 819GB Free (931GB Total) 5.9
Windows 7 Home Premium

I have 8438MB total graphics memory
512mb dedicated graphics
0mb dedicated system
7926MB Shared

Graphic Card: ASUS EAH5450 Series

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