Last time he doubled he went from 2000 to 4000 per day. If doubling again, does that mean going from 4000 to 6000 or 4000 to 8000 (doubling would mean 8000)?

We're doubling from ~4000 per day to ~8000 per day.

That's great.
We should all get an ample supply now.

Each arp unit requires about 800mb ram and up to 1.2gb of storage, though it will usually be around half that (700m) - it only uses the full amount when checkpointing. Each unit will read about 5-6 gb from disk and write 7-8. Disk activity is heavy when checkpointing, and limited otherwise (though significantly more than say mcm, which will write only a megabyte or two per entire unit).
I estimate downloads to be about 60-80m, and uploads to be 90-120m or so - I haven't gotten enough units to be certain on transfer numbers.

As long as ARP projects are not the only project in the mix, BOINC manager will give you this outcome automatically.
Other than a Linux machine (which should focus on running Smash Childhood Cancer), it's always beneficial to the Grid as a whole for computers to accept all units.

Perhaps Mapping Cancer Markers needs a 200, 300, 400, 500, 600, 700, 800, 900 and 1K badge to keep badge hunters from getting bored with it.