A pair of 460's would be nice and quiet. A pair of 560 Ti 's or even 570's even better.
SSD is not a bad idea. You could get a nice 64GB SSD for ~$100 on sale. Then get him a large HDD, say 1TB for $60. Put the OS and install all the apps on the SSD, and then set it so his storage drive for pictures, movies, music, is all on the HDD. Way awesome speed and way awesome storage capacity that way.

I'd look at a Sandy Bridge i5 2400 for budget or go the full montey with Sandy Bridge i7 2600K for $100 more. For mobo, we need some more info for recommendations. Will you ever be tempted to run 3 GPUs, or will 2 be the max?
You can set all kinds of flags for the latest BOINC client, like make it stop crunching when a certain .exe file runs, set it to crunch only at certain times of the day; only when idle, etc...lots of ways to make the rig really snappy for Pops, but still get millions of good credits too!

The number of GPUs will help with the case and PSU recommendations as well. You can go quite a bit cheaper than a HAF-X and 1000W if you are doing 2 GPUs. But for 3 GPUs, that is what I would recommend.

I would also say 8GB of DDR3 1600 RAM at this time. He would not notice a difference with any more RAM for what he's doing; so I would be going for 8GB and throw the extra $ at the better CPU or GPU.