Depending on what is running around the house, between 3.2Thash and 3.5 Thash.
I don't run my nvidia cards, ati4850's or CPU on mining, they're doing something else - they might add another 300-400 Mhash. And no, I didn't buy a whole stack of stuff, that just happens to be what I have. (and I don't play games on my computers either - only sometimes on the laptop)
How? What are you doing to just have thoes "happeing to sit around"?
If I had to guess, he's either putting in with a distributed computing project for BOINC, or (more likely) does infosec work and uses them to crack stuff on the GPU. Both bitcoin and GPU cracking for security are basically the same thing, but there are a number of tools that only run on CUDA. I got a dual-card setup for the same reason.
The prize goes to Canadian Kodiak. I swapped over from crunching boinc - if that community decided to swap, difficulty could double overnight. I was doing 1-2M credits/day there on a mix of projects on the "Siciturastra." team but my processing speed is only around #190 globally. I've left four nvidia cards and two 4850's crunching there because they go slow on mining.
I find the o/c community interesting (including on this board) because GPUs fry. I have had two dead 5970's this year and also have a graveyard of a 2600 ( a fun little card), HD5850, a 4890 and a GT8800. All running stock speeds. Overclocking cards shortens life, so I will watch with interest when the 24/7 guys start breaking theirs. (I also had one of my slower machines die yesterday - mobo blew a resistor on startup, but I've been lucky and only had one PSU go bang)
Currently I have an i7 in the laptop I use most of the time, six other i7's (various - with a spare 920 cpu ready to go into a mobo this week when it arrives), and AMD thing somewhere and a 9450 quad on my daughter's desk with a 5850 in it. She doesn't mine with her laptop (the 5850m is good for 125Mhash). I have other stuff lying around unused - eg, I have two P3 servers with nice fibre connected raid enclosures in the rack but they are noisy to run, and a couple of laptops in a box (the D620 is still very good but got retired). There's a netbook and wife's laptop around too.
That doesn't seem excessive.