There is no such thing as 5.5V AGP. AGP 2x had a 3.3V signal strength. AGP 4x had a 1.5V signal strength. AGP 8x had a 0.8V signal strength.

I've no idea of 5.5V AGP slots... Early generations (1x-2x) are 3.3V, while later generations have lower voltage (4x @ 1.5V, 8x @ 0.8V).

If you're referring to Pentium 4 motherboards... I think they commonly come with a non-universal AGP 4x slot (blocking 3.3V only cards), with later chipsets feature AGP 4x/8x slots.

EDIT: The only known mention of a 5.5V AGP slot is here, though I'm not certain whether it was a typo or some other reasons. The OP of that post there was talking about upgrading his existing Dell 8250 (probably this one, Dell Dimension 8250), which used Intel 850E (Socket 423/478, RDRAM interface).

The only P4 boards with 3.3v universal AGP slots I know of are rare early SiS or VIA chipset motherboards, and they're generally not very good. The via boards only support FSB 533 CPUs, and so do most SiS boards with a few exceptions. If you're looking for a fast, accessible 3.3v agp machine to stick your v5 in, get a socket A VIA KT333 and Athlon XP 3200+ FSB166 CPU (2333MHz).

In case you want to run a Pentium 4 with a 3.3V AGP slot you can look into motherboards that use the SiS 645DX chipset like ABIT SA7 and DFI SIS 645DX. All boards with the 645DX chipset are able to run a 133MHz (QDR533) front side bus. I have no hands-on experience with the motherboards I mentioned but I don't see any reason to doubt their functionality and stability.

Sorry I should have put 3.3V 😜 the SIS 645DX chipset looks to be a winner. Thanks for the help 😀