Well, I did a quick and nasty search with +geforce +vddq on google, and I found something that might help a little.
I have a VGA card with GeForce2 mx400 64MB ,but my MotherBoard is P4 board that only works with a AGP 1.5V card ,Is the card compatible with the MotherBoard?
1, Such 1.5V, 3.3V are not core voltage, but VDDQ (a kind of control voltage); AGP 2X card uses 3.3V VDDQ, while AGP 4X uses 1.5V VDDQ.
2, The AGP slot (compliant with AGP V1.0) supports 3.3V AGP card; while the AGP slot (compliant with AGP V2.0) supports 1.5V AGP card.
3, For new motherboard, the AGP slot complies with AGP V2.0, that is,it offers compatibility with AGP 2X and AGP 4X cards. But for the present P4 board, it only provides 1.5V. When using AGP 2X card (3.3V VDDQ), the board or card will be damaged. When using AGP 4Xcard (1.5V) or certain AGP 2X card (with adjusted circuitry) such as our PV-S03 card, it will operate normally.
4, TNT2 family, GeForce family, SiS 315 etc. VGA card are all AGP 4X card, so they all can work smoothly on P4 board.
The one in bold is what we're concerned about. AGP 4X is using a VDDQ of 1.5 V. Which software are you running that monitors this voltage? It could be that the the high warning for this voltage could be set at 1.55 V, and the 1.56 V is probably triggering the message. You may want to look for the configuration and set the high level warning for VDDQ to something like 1.57 or 1.58 V. But, I don't think you have anything to worry about with it being 1.56 V.
URL for the information: