I currently own a BFG 8800gt OC. Would it be okay to put a Evga 8800gt in there, seeing as there prices are lower?

Yes it will be okay. However the clock speeds will revert to the slower one, which in this case is the EVGA. That means the OC on your BFG will be negated.

so there is no way you can OC with SLI?

I think you can. The limiting factor would be the one that doesn't OC as well as the other.

http://www.tomshardware.co.uk/foru [...] sfire-faqs
 
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 stable.