I recently upgraded my GPU from a ATI Radeon 5770 HD to a EVGA SSC GTX 960 4gb. 
I am now considering going SLI with a Zotec 960 2gb.
I am wondering if it's worth it and if I would have to upgrade my PSU which is currently at 480w.
I've never had a crossfire or SLI setup and I'm wondering what some of the pros and cons are? Or if I should just hold off and upgrade my CPU and MOBO first? 
Any feed back will be greatly appreciated!
 
Rig:
Mobo - stock dell POS
CPU- AMD Phenom II x6 1035T @ 2.7
GPU - EVGA SSC GTX 960 4gb
Ram- 12gb @1333

For SLI, both GPU's need to have the same amount of Vram.  Both 2GB or both 4GB.  Different manufactures are fine.

You're welcome...

dwoodward

bdary
For SLI, both GPU's need to have the same amount of Vram.  Both 2GB or both 4GB.  Different manufactures are fine.

Is this really a thing? I remember way back when having a 8800 GTS 320 and a 8800 GTS 640 and SLI worked fine. Granted that was years ago lol.

It is a real thing. Just look it up on Nvidia.com under sli requirements. You can use a program called Different SLI to supposedly overcome this, but why would someone buy a lesser version and degrade the performance down to 2gb on a 4gb card? The 4gb card would be slave to the 2gb as they have to share the same information, which would cap at 2gb.

Furthermore, the price difference from 2gb to 4gb is typically $20-30 usd.

bdary
Here you go.  SLI requirements  >>  https://forums.geforce.com/default/topic/469402/
 

Thanks bud. Can't do much searching on mobile while at work lol.

Here it is: straight from Nvidia site: VRAM & Bus Width MUST match
 
Thanks for the link bdary
 
Scarlet: see the Note - used to be possible - re diff. VRAM Nvidia has change the rules on that
 
All GPUs must have the same amount of VRAM and an identical Bus Width
-- You cannot use a "2GB" version with a "4GB" version or "3GB" version with a "6GB" version in SLI or a 1GB 192 bit bus with a 1GB 256 bit bus Etc.
Note: It used to be possible to enable SLI on cards with differing VRAM but this is no longer available.
   Note: In SLI the amount of usable VRAM does not stack. It works like a RAID1 hard disk array where data is mirrored between each GPU. For example, having 3 GPUs with 4GB VRAM means you will only have 4GB available, not 12GB.

dwoodward

bdary
For SLI, both GPU's need to have the same amount of Vram.  Both 2GB or both 4GB.  Different manufactures are fine.

Is this really a thing? I remember way back when having a 8800 GTS 320 and a 8800 GTS 640 and SLI worked fine. Granted that was years ago lol.

I had that exact same setup for awhile...  Seems to me that was when SLI first came on the scene.  I could be wrong, but that's my first memory of SLI.

Adeyotol
I recently upgraded my GPU from a ATI Radeon 5770 HD to a EVGA SSC GTX 960 4gb. 
I am now considering going SLI with a Zotec 960 2gb.
I am wondering if it's worth it and if I would have to upgrade my PSU which is currently at 480w.
I've never had a crossfire or SLI setup and I'm wondering what some of the pros and cons are? Or if I should just hold off and upgrade my CPU and MOBO first? 
Any feed back will be greatly appreciated!
 
Rig:
Mobo - stock dell POS
CPU- AMD Phenom II x6 1035T @ 2.7
GPU - EVGA SSC GTX 960 4gb
Ram- 12gb @1333

Most likely yes, sli will be bottlenecked by that cpu. With the stock Dell mobo you most likely cant overclock the cpu, which you would need to do because you need to get to a consistent 3.4ghz on the cpu to get the most out of a sli setup. Your cpu only goes to 3.1 on a boost. Since your using the stock Dell mobo im going to assume your using the stock amd cpu cooler, or something very similar, which is def not going to be adequate for overclocking anyway, not to mention the mobo voltage regulators are most likely not designed to take that kinda load. Im not sure what case your using, stock Dell?. but like CoolGTX mentioned it probably wont be able to cool that kind of setup anyway so at the very least youll have to fanmod it. Not sure what the psu make/model is but 480 watts for that setup with sli is cutting it too close and gives you no overhead. Youll be working the unit at close to its max capability and even if its from a reputable brand it wont last long with that kind of work. Im not saying it couldn't boot up and run because it will but when you put a full load on it may not hold the load, you may see all kinds of weird software errors starting, bsod's or it may just shut itself down instead of blowing up. Imo, you would need a new one to run sli and id shoot for something that's capable of running the whole setups full wattage need at between 50 and 70% of the new psu's full wattage, which is where all psu's perform at there best and last the longest. I would put that money toward a cpu/mobo upgrade at least before sli. If you can get the right mobo and cpu you can really push the gcard you have and you will probably see a big difference in its abilities. A SSD for a boot drive, 60gb at the very least, would be very helpful too, and that goes for the pc you have now or a newer one. Hope that helps. :)