Off the top of my head:
Direct X (and equivilent) - In the old days, game developers used to have to manually write drivers for sound cards and other computer components. This technology provided device-independence.
Graphics Cards - Reduce load on processor / RAM
Physics Cards - PhysX and others, basically uses the same principles as the graphics card, reduce load on RAM and processor
Bus Widths, Bandwidth, Etc - Get data around that motherboard fast!
Processors - Faster! HyperThreading! Quad-Core! Etc. Get more tasks done in each game cycle - more sophisticated graphics, AI, etc!
Memory - Hard to believe the days of Win3.1 where 8MB was godly!
BroadBand Internet, Switches, etc - Faster internet = online games, new networking potential
Programming Languages - Java/C# Platform-Independence, Virtual Machines, Garbage Collection, etc
Ergonomics - PC gaming devices such as controllers, joysticks, headsets, etc.
Sound Cards - realistic sounds, surround, etc.
Personally, I believe that buisnesswa, academia and the military contribute more to pushing the edge of technology. Most of the "great advances" in PC hardware were simply making existing experimental technology economic. Making it affordable for everyday users.