I actually wondered the same thing myself. So, I went out and upgraded from 1GB to 2GB, while still keeping dual channel and similar timings of RAM.
I noticed an overall quicker response from Windows. Things like bootup, opening Windows, and overall multitasking was improved.
However, although some games are getting hungry for RAM, I have not noticed a gaming performance increase. That's not to say you wouldnt benchmark higher, but in terms of noticable difference to a human (or Moderatorbot) theres little if not none.
If I were to build a gaming rig for someone, I'd probably end up sticking 2GB in there. For the responsiveness and future considerations.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64