ATI HD 5770 GRAPHIC CARD

Yes it is compatible! The interface on the card has nothing to do with the system being 64bit.

Here is my problem, am building a mid-range gaming computer for my nephew.
when the ATI 5770 is installed in the PCI-E X 16 slot (with the 6pin power connnected) the system turns on and off every 3 seconds, They told me that I needed a better PSU, so I installed OCZ MODX STREAM-PRO 600W, I have the same problem.
When I stick the card in the PCIe X 4 slot, it WORKS.
please help.
thanks.

First of all,its PCI-E X16 not CPIe,i edited your posts  
Anyway,do you have any other cards to test it on the PCI-E x16 slot ?

Yes, it sounds like it could be a faulty motherboard. See if you can try it on another one and if it works you have your problem.

Mobo's are fairly cheap in the low-end these days, should be able to pick up a decent mobo for $50~$100 US I imagine...

You can try to update your bios also, that usually fixes problems. I do not know if it will fix yours, but it's a try.

Some 5770 that are PCE-E 2.1 have bugs on an older slots like PCI-E 16x. So the update might fix your issue. Also i recommend as well to test another card on that system. And try the 5770 in another system as well, if it works there, the cad is fine.