I have an 8800 gtx 758mb.
 
Will it SLI with a 8800 gt 512mb?
 
Thanks.

Nah, you need a matching card to do that, so another 8800gtx 758mb.

No.  
You can only SLI matching card models i.e. 8800GT 512 w, 8800GT 512
but they can be different brands BFG 8800GT 512 w, XFX 8800GT 512
only crossfire is where you can pair different cards

Nonono..............

http://www.tomshardware.co.uk/forum/forum-15-208.html
 
Do the cards have to be same model?  
I answer it with an example:  
You have a NVIDIA Geforce 7800GTX 256, and you want to add another card , the second card  has to be 7800GTX exactly.  
 
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 )  
 

Simply NO
2 cards must be the same

Definetly not. You need to have identicle cards eg. 8800GTX with 8800GTX. You  
can in theory add an 8800GTS though but the faster card will handycap its core/memory to match the slower one.