Hi Martin.

I see you signed up in 2009. It depends on your hardware config and your definition of "old computer". Some people consider 2yrs to be an old computer, some people 10 years to be so.

Actually, sometimes project admins increase the workload on single units to better fit the increasing processing power of newer computers so it usually brings less traffic to their servers. I don't know if they did it over here at WCG, but at least consider this to be possible.

As far as crunching for WCG is concerned: If you run any hardware with less powerful than a core2-CPU, maybe just put your computer aside for backup or just give your computer to charity and get yourself a cheap computer with an AMD A8 APU or a cheap Intel-CPU and a discrete graphics card (sadly WCG does not offer any GPU-supported project these days). Maybe get yourself a cheap "Computer Upgrade Kit" on ebay?

If you have to pay for your own electricity, it's worth chancing the crunching system anyway. If you consider that your CPU (depending on Boinc-Setting) usually uses 100% of its allowed power, it will make a difference if you choose a brandnew lowpower-CPU. Or you can see it the other way around: For the same electricity consumption, you may be able to provide WCG with way more results.

General example about average benchmarks:
According to Passmark / CPUbenchmark.net an 8yr old Core2 Quad Q6600 is getting 2987 points with a max TDP of 105 Watts, while a brandnew dualcore i3-6100 is getting 5573 points with a max TDP of 51 Watts. So for those 105 Watts consumption of the Q6600, you even could operate two i3-6100's (putting aside other parts in the PC-tower), so in 8 years of evolution, that would have meant 3x more powerful than initially. This was just generally speaking.
And by the way, the introduction price of the Q6600 on Jan 8th 2007 was 851USD. When i3-6100 got launched last september (2015), its introduction price was 117USD. (Data collected at cpu-world.com)

You can exactly look up what results each CPU provides within the WCG crunching network by looking up:
http://boincstats.com/en/stats/15/host/breakdown/

You can play with those charts to find out where your exact CPU is listed. Their site show "3815" different kind of processors (while some listed many times due to different steppings or manufacturer cpid's)

Good luck! ;)
-chris-