simple question: 2 video cards

Yes, but SLI will run at the lower clocked cards settings. I would stear clear of PNY. EVGA is the way to go.

Yes. can run 2 cards from 2 different vendors, although I think running 2 from the same vendor is better.

Already answered in FAQ:
http://www.tomshardware.co.uk/forum/245454-15-crossfire...

But what about the memory? Can you add a 7800GTX 512 and use that with 7800GTX 256?
It's recommended to use a 7800GTX 256 with a 7800GTX 256 and 7800GTX 512 with 7800GTX 512.
However you can use a 7800GTX 256 with 7800GTX 512 , BUT the 7800GTX 512 will lower its clock to operate with the 256 one , so it wont have its true power. (This option isn't recommended )


What about brands ?
Well the brand doesnt matter , again for example , you can use a XFX card with a EVGA card , or a SAPPHIRE card with a DIAMOND card , just make sure they have the same memory and same clocks.

What will happend if 2 cards have the same amount of memory but one has higher clocks than the other?
One of the best ways to learn is to make examples.
You have a 8800GT(Normal edition,no OverClocks) and you want to add another 8800GT(Which is OverClocked),then:

1_The OverClocked one will lower its clocks to reach the Non-OverClocked card.

2_You can OverClock your card(The one that has Stock frequencies) and reach the OverClocked card.
Remember, you cant ALWAYS OverClock the normal card to reach the other card,because it may not OverClock that much and may become unstable.

SLI just isn't that flexible.

So why did my standard clocked 7900GT raise it's clock speed to match the OC'd card that was in the primary slot?

In what way?

Compare it to the Crossfire mousemonkey

The OP was asking about SLI not Crossfire, and whilst Crossfire appears to scale quite well the reviews I've read seem to suggest that it still returns lower fps's than an equivalent SLI setup when AA and AF are cranked up.

what? are u sure ? i havent ever heard that  

Everest would report my Asus 7900GT as a 450mhz GPU when it was on it's own but as 550mhz when the factory OC'd XFX 7900GT was added.

Thats very strange but ok i may add as an exception to the FAQ

Another Q: what happens when u pull out the OC'd one ? will the ASUS one have the XFX clock or it will reduce itself to normal?

It would go straight back to it's original clock speed of 450mhz, the XFX card which had a clock of 550mhz would OC to around the 600mhz when on it's own but when paired with the Asus card the highest I could get was around 555 - 560mhz before things went tits up but the Asus card on it's own would not get any higher than 520mhz or so before going off to la-la land, it's the little fun things like that which can only really be discovered if you run and experiment with an SLI rig and even then no two setups will always behave the same regardless of how seemingly identical they may appear to be.  

ok thanx alot mate   will add it to the FAQ

Thank you all for your help