I want to buy a new mobo to put my 8800GT 512 with another one in SLI, but the 780i is too expensive for me. Which one would you guys recommend?

750i. Don't consider 600s nvidia chipset, they are trouble, and not worth the headaches.

I can't find any memory that supports the nvidia 7x chipset. Shouldn't they all work on it anyway?

I agree on the 750i. You save yourself 100 and still get the 2 PCIe 2.0 slots even though if you SLI one is x16 and one is x8. Another thing to consider is most of the 750i's have a 800mhz memory standard while the 780i usually allows 1066 no problem.
 
 
Not sure why you can't find memory to support either the 750i or 780i chipset.. here is an example http://www.evga.com/support/mbmem/  
 
It's always best to go with manf approved memory

what do you mean that you can't find any memory that supports 7x chipsets, 750-780 is DDR2, and 790 is DDR3.

I mean that when I look at the memory specs it says on it:
M/B Chipset : X38, P35, 965.  
One i was looking had all that plus it also said nVidia 6x i series.  
 
So I figured that if it didn't list the 7xi, then it won't work on it.

those chipsets that you mentioned aren't nvidia chipsets those are intel chipsets.  X38 is DDR2, and the X48 is DDR3, but if you are wanting an nvidia chipset it is pretty much DDR2 RAM unless you go with the 790 which is DDR3. I hope this helps