We use the Dell Optiplex 9020 i7 at work and they still perform well.  I was looking for the same system to build out an inexpensive gaming box.  I chose to buy this refurbished system from TekRefurbs and I recommend them.  The system arrived well packed and spotless and works perfectly.  It came with a new usb keyboard and mouse, USB WiFi dongle, a welcome packet and a mouse pad.  Nice touches.In order to use a good video gaming card I replaced the power supply with a higher power ATX supply using a 24pin to 8pin adapter, also purchased from Amazon.  The system is ready to go and powerful enough to keep up with current software and games, despite that it's a few i7 generations old now.  I'm very pleased.