Dimension 4600 / 7600GS AGP - No video

Haven't been here in awhile.  Can't say as I like this new forum so much; extremely hard to navigate.  But, oh, well.

So, this card will function in the Dimension 4600 - mine is the P4 flavor, 2.8Ghz, 2.5 gigs ram, brand new 400 watt PSU w/20A on the +12v rail. 

I have a problem with my factory fresh Asus version of the 7600GS/ 256 megs, which I bought to replace a GeForce 6200/128 meg card because I need more card ram.  The 6200 worked perfectly and has taken up residence in a homebuilt system.

I took the 7600GS for a test run in the homebuilt system first, just to be safe as I've never seen a video card that required power from the power supply.  Kinda scared me a little. 

It worked like a charm.  Practically installed itself. And I didn't uninstall the existing ATI Radeon 9200 drivers, which was the only video card ever in that system.

Put the 7600GS in the Dimension - NO VIDEO SIGNAL.  Yes, I attached the molex power to it  and watched the card's fan spin up (Man!  It's dead quiet!)

Pulled the 7600 out of the Dimension.

Tried the onboard video - I have video.

Went into the BIOS; AUTO detect is selected for the video source, right where I'd set it when I got the 6200 for it. 

Put the 6200 back in the Dell; I have video.

Pulled the CMOS battery.

Reattached the onboard; booted up, checked the BIOS again, left the video at AUTO detect.  Video is present.

Back to the 7600.

Switched some molex connectors around so the 7600 would be on one by itself.

No video.

I didn't uninstall the previous Nvidia drivers because they're the same as the 6200's. Besides, I get no video in the POST screen even.

The homebuilt system that will run the 7600 has the same brand power supply, but it's 350 watts.

My internet service is wireless - I connect through a router via ethernet so no modem or other add-in cards.

The only difference I see between the 6200 and the 7600 is the AGP keys.  The 6200 has both the 3.3v and the 1.5v while the 7600GS has just the 1.5v key, up next to the card's hook.  My 7600 is definitely AGP as there is no PCI-Express slot in my testbed homebuilt system - it's an MSI K7N2 motherboard.

The 6200 is the only video card I'd ever installed in the Dimension; used the onboard before that.

Running XP Pro SP3, current on all updates.

What am I missing?

***New discovery. 

My Dimension's new PSU is the switching type.  After I shut down any computer, I ALWAYS turn off the PSU (if it's a switching supply), then unplug the power cord from the wall and wait until the motherboard's green light goes dark before I open up the box to work inside it.

With this Dimension and the 7600 installed, I plug it back into the wall, flip the PSU switch to ON, and the Dell immediately starts to boot on its own.  I do not touch the front panel power switch.

With the 6200 card installed in the Dimension system, I have to press the front panel power button to start the machine.

Both cards have fans, but the 6200 does not require an additional power source.

The homebuilt system also has a switching power supply, and I go through the same power off/on steps with it. 

So how is it others with Dimensions and this 7600GS AGP don't have this problem?