The right time to upgrade: 2014
This is for high end at least.
The next leap in gpu and cpu architectures seems to be in 2014:
The last time this happened was in 2006. Compare the highest end hardware of 2005 to that of 2006:
CPU: Amd Athlon FX-60, worth $1000+
absolutely trounced by: Intel Core 2 Duo E6400 worth $400
GPU: nvidia Geforce 7950 GX2, supports dx9 only, worth $600+
absolutely trounced by: nvidia Geforce 8800GTX, supports dx10, worth $500
A top of the line 2005 machine, worth $3000 was relegated to mid/low end in the span of a year, and is now totally worthless performance wise.
Meanwhile, a top of the line 2006 machine is still relevant today (Core 2 Extreme quad core, 4GB ram 8800GTX)