The only time I felt like I've needed an upgrade is while playing Planetside 2, which is/was very CPU bound for my setup. However, when it was initially released, Planetside 2 ran like a three-legged dog even on some higher end rigs. It's much better after a few rounds of optimizations by the developers, with more scheduled for the next month or two.
I dual boot Linux boot on the same machine for my day job, 5 days a week all year. For this purpose it has actually been getting faster with time as the environment I run matures and gets optimized.
As good as it is now, I remember struggling to keep up with a two year old machine in 2003.