For my main rig, I won't upgrade it for folding unless something self-destructs (e.g. went to Radeon 4870 when the 4850 became unstable). I also won't go nV on the GPU until they implement DX11 or DX10.1...
I plan to upgrade my home server when the Core i5 price drop ripples through the market, because right now it has a very poor PPD / watt ratio. The cheap PSU didn't do its job, so the rig is still alive and kicking after ~2 years of folding / seeding torrents

Home server - Pentium 4 2.4 GHz Northwood: ~100 PPD
ONE CORE of my laptop's C2D T7500 at 1.2 GHz out of 2.2GHz: ~200 PPD
If I run Linux SMP on the laptop (GPU is the defective nVidia G86 so there's no way I can fold on it), I can generate 1500-2000 PPD, likely drawing half the watts of the 100PPD Pentium 4.