I'd convince them it's an excellent hobby/learning experience.  Plus you can always make the financial argument.

3 5850's will use approximately 450-500W continuously while mining and your machine will add a bit more, but let's assume 500 for the sake of argument.  That's about 12 Kilowatt Hours/day * 30 days or 360 Kilowatt hours/month.  Multiply that by your KWH rate for electricity and you can guesstimate how much it'll cost.   So, at 10 cents a KwH you're looking at $36/month in electricity.

Those cards can pretty easily do 330-340 MH/sec, so let's say a flat 1 GH.  Using that against the current difficulty with $14 BTC prices (both of which are subject to change), you can expect to mine about 18 BTC or around $250 worth of BTC.  

Of course, you now have to deal with a loud/hot box in whatever room you stick it in and all the hassle of getting BTC sold and into your hands as cash but it's definitely worth it.

Aside from the PCIe x16 slots that we all keep our super extreme graphics cards in, what is the best use that you have found for your other available slots? This includes plain old PCI, PCIe x1, x2, x4, x8 and also PCI-X (eXtended).

Adding USB3.0 functionality to my mobo has been a good experience. Single-lane PCIe bandwidth facilitates the speed of the new USB standard.

I'm interested to see what gets posted here. PCIe x16 for graphics receives the overwhelming share of attention but the other types of slots can be just as handy.

Given what's in the pipeline, it's more likely going to be this way if bitcoin is indeed an AMD marketing campaign

"AMD proudly announces the success of their 2 year old marketing campaign called 'The Bitcoin Project'. The project has helped propel sales of existing 5-series and 6-series card while creating financial value for AMD's users. To return even greater value to users and increase AMD profits, the new 7-series Southern Island GPUs will be available in retail channels on 1st August. Featuring 15% lower power consumption and 25% better hashing performance, it is expected that inventory will be cleared by Bitcoin Project Users as fast as they can be shipped."

cute..  they even went so far as to start with CPU mining so as to avoid suspicion.

This is why conspiracy theories, marketing and religion have such power..  in pursuit of a picture they've already envisioned in their mind, they join unrelated dots like a preschooler, and insert dots like a woodpecker on acid.

With most pools, your reported hash-rate is based off of shares over time.  GUIMiner reports the 'actual' hash-rate.

If the pool is reporting a higher hash-rate than what your 'actual' hash-rate is, you're getting 'lucky' solving each share.  This is normal variance.

Cheers,
Kermee

The best bang for buck in the 6xxx cards is going to be a 6950 or 6970 depending on prices I'm not from australia so I'm not familiar worth who will ship to there our your local stores.

The 5850 is a good card for mining so I'd recommend going for another 5850 and the Seasonic 500W (Depending on model) should be good enough for two of those... not recommended for long term but I've done 2x 5870 in Crossfire on those before for gaming.

Depending on your purpose for mining, you might want to see how the price of bitcoin stabilizes in the next few weeks before pumping more money in. If it dips sharply and you're not able to simply hold on to your bitcoins for the long term, it may not make sense to invest more.

2x5850 would also be fine for gaming in crossfire mode but tri-fire don't give you that much more and you'll definitely need a new PSU to handle that.