my computer always lags on pc games and i know it has the qualifications. i don't understand.?

i try to play games on my pc like left 4 dead 2 and other games like that. i go on systemrequirementslab.com and it says i can play the games i want at above recommended settings so i really don't understand what i need to do. is there something i need to do to be able to access my computers abilitys and such? i will give you an example of the specs i have.

CPU

Recommended: Intel Core 2 Duo 2.4 GHz

You Have: AMD Athlon(tm) II X4 630 Processor

PASS

CPU Speed

Recommended: Intel Core 2 Duo 2.4 GHz

You Have: 2.8 GHz

RAM

Recommended: 1 GB for XP / 2 GB for Vista

You Have: 3.7 GB

PASS

OS

Recommended: Windows® 7 / Vista / Vista64 / XP

You Have: Microsoft Windows 7 Home Premium Edition (build 7600), 64-bit

PASS

Video Card

Recommended: DirectX 9 compatible video card with Shader model 3.0. NVIDIA GeForce 7600, ATI Radeon X1600 or better

You Have: RS880

PASS

Features: Recommended attributes of your Video Card

Required You Have

Video RAM 128 MB 1.9 GB

Hardware T&L Yes Yes

Pixel Shader version 3.0 4.1

Vertex Shader version 3.0 4.1

Sound Card

Recommended: Yes

You Have: Realtek High Definition Audio

PASS

Free Disk Space

Recommended: At least 2.5 GB of free space

You Have: 580.0 GB

PASS