I have a similar problem to the poster in this question: New GPU power supply woes (ASUS GT640 + HP XW4600)
I also have a HP XW4600 and the gfx card: ASUS Nvidia GT640-2GD3
The card doesnt seem to have a connector or require a separate power supply, and when plugged in, the machine fails to boot, with 6 beeps in a cycle. The fan on the gfx card starts running though.
The XW4600 Mainboard has printed on: SLOT2 PCIe2 x16 75w + 75w. I have also tried in SLOT4, which has the same print.
Pictures of the card:
Update: I contacted ASUS regarding the card:
Your card uses 65W and is within the power requirement of the pci express slot so that t does not require additional power. The system itself you are using is a bit on the older side and it might possibly need a bios update to get newer type of hardware to function on it. There might be some microcode or some other enumeration issues in the bios that creates this issue.
Can you verify that the card works in another computer?
I have updated the BIOS but still it fails to boot.