Amazon.com: Dell OptiPlex 755 MT/Core 2 Quad Q6600 @ 2.4 GHz/8GB DDR2/500GB HDD/DVD-RW/Windows 7 PRO 64 BIT : Electronics
Refurbished: Dell Optiplex 755 Small Form Factor Desktop (R2/Ready for Reuse) with Intel Celeron 430@1.80GHz, 2GB RAM, 80GB HD and licensed Windows 7 Home Premium from a Microsoft Authorized Refurbisher - Newegg.com
How to upgrade a DELL OptiPlex 755 USFF PC - YouTube