New power supply problem... HELP..

hi

I recently upgraded psu to a antec signature 650w. I pulled out my old 400w psu and installed the new psu hooked up every thing and turned it on ... i got no post.
I have a agp HIS 3850 card installed in my abit IS7 mobo. I pulled out the video card and replace it with my old ati x800 card and turned it on... it posted and booted normally.

I then removed the x800 card and re-installed my 3850, turned it on and no post.
I disconnected the 20pin atx and the 4 pin p2 plugs and connected the two plugs from my old psu, i left the video cards power connector connected from the new psu, i ran the new psu to power up the video card, then i powered up the old psu to power up the mobo and it posted and booted normally.

I disconnected my old psu from the mobo and reconnected the 20 pin atx and 4 pin p2 plug from the new psu and turned it on and no boot.

works with new psu and x800 card
works with old psu and 3850 card
works when the new psu is powering the video card and old psu is running the mobo

dosnt work with new psu running card and mobo

what do you guys reckon? because i am stumped.

cheers

the video card was fitted.

connected the old psu to the motherboard, switched it on and booted

(didnt touch the video card)

then removed the old psu and connected the new psu
turned it on and no boot. just 1 long and 2 short beeps.

cheers

yes it was connected properly i have spent easy 5-6 hours troubleshooting and testing and swaping psu's .

yeah i thought that too but why does it work with the x800 but not the 3850, there's the real question.

i currently have the system running with the 3850, i have the antec signature running the hdd's and cdroms and video card and a 300w running the mobo.

What r_manic said. Except the difference in power consumption is only about 3 amps, so your old PSU should work with your new video card.