Easy, get the Recommended Specs for the games you want to play then compare to the machines available.
Example:
Skyrim wants
-Windows 7/Vista/XP PC (32 or 64 bit)
-Quad-core Intel or AMD CPU processor
-4GB System RAM
-6GB free HDD (Hard disk drive) space
-DirectX 9.0c compatible NVIDIA or AMD ATI video card with 1GB of RAM: Nvidia GeForce GTX 260 or higher; ATI Radeon HD 4890 or higher
-DirectX compatible sound card
-Internet access for Steam activation
GTA: IV
Recommended System Requirements
-OS: Windows Vista - Service Pack 1 / XP - Service Pack 3
-Processor: Intel Core 2 Quad 2.4Ghz, AMD Phenom X3 2.1Ghz
-Memory: 2 GB (Windows XP) OR, 2.5 GB (Windows Vista)
-18 GB Free Hard Drive Space
-Video Card: 512MB NVIDIA 8600 / 512MB ATI 3870