AGP 2.0 is really AGP 4x and agp 4x will work on an agp 8x slot. but you are not upgrading cuz your goiong from 8x to 4x. makes no sense for an upgrade.
My guess is that your current motherboard either has a universal AGP slot or an AGP 2.0/3.0-only slot. As long as the new graphics card supports 1.5V signalling voltage, everything should be fine, but a word of caution is that I have encountered cards before which only signal at 0.8V (AGP 8x-only cards).