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Hey everyone. I'm building a monster rig with 4 5770s. The main problem is power. It seems far more cost effective to strap 2 600W PSUs together on the same mobo. But does anyone know how? I mean it must be more difficult than just plugging them into the GPUs yes?

Only a bit more difficult. The main problem is getting the secondary ATX PSU to turn on, since the motherboard won't be connected to it. You can use something like this:

http://www.techpowerup.com/articles/other/22

You'll want the secondary one to turn on before the system starts booting up too.

Hey thanks buddy! I wasn't sure that it would work.. ha ha sweet!

http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=34_186&products_id=21193
http://www.frozencpu.com/products/5637/cpa-167/Lian_Li_Dual_Power_Supply_Adapter_Cable.html

The best way is to get an adapter cable that turns the slave PSU on and off with the master.

Radeon 5770 has a TDP of a bit over 100W, so with a good 600W PSU you should be able to run a system with four of these cards without problems, no need to patch two 600W PSUs together...

do you get any grounding issues with using two different psu's?

They're both grounded at the socket, so no. It's good practice to have them share a ground connection though.

It's pretty easy hack them together with just a little bit of wire. All you have to do is draw 1 wire (pin 14, green wire) from one ATX connector to the other (where the "on/off" signal is generated). The PSU knows to turn on when this wire's voltage goes to 0 (ground). The adapters also include a ground wire (black) which isn't strictly necessary, but does insure both PSUs are grounded correctly (safety thing, not strictly necessary). You can draw this wire as well since it's about a 2 minute job.

You just have to short the green wire and black wire on the ATX connector. I usually just use a paperclip. Then put a little bit of electrical tape over it. Just make sure never to run the second PSU with no load on it. Make sure there is always at least a fan on it.

Also 4x5770 doesn't need that much power. I have 2x5830s and 2x5770s on one computer with a Corsair TX750. I am pushing that power supply to the max. But if you wanted a bit of headroom you could grab an 850W reputable brand PSU and go to town.

I remember doing something like this with the old AT style PSU's. You just had to hit both power supplies at the same time or half your hardware would work!  :D

Is it safe to use second PSU to power GPU? Should I turn on PC first and then turn on PSU for GPU? Or in the reverse sequence?

I have a rig that has two 200w PSU's set up this way, i didn't use any fancy connector or anything like that.  I simply shorted the green and black wire on the second PSU to make it turn on, it's connected to the second video card only.  Rig has 2x5770's.  Why waste money on another PSU when you have extras sitting around?  Just make sure that you turn on the second PSU before the main PC, as long as you get it within a second or two you will be fine.  I usually insert my paperclip and then flip the switch on the main PSU, works like a charm.

I normally do both simultaneously; make it seem as if I have a single big PSU.

You can short the second psu to turn it on, should have no problem using both. Although like someone else mentioned, you can run 4x5770 pretty easily with a good 650w or 700w.