I performance increase between AGP 4X and 8X isn't that great of a jump, and 4X should really be sufficient. I haven't seen your other components here (maybe I just missed them), but I'm sure you could find an updated motherboard for not very much (40USD maybe) that would still accomodate your old hardware, especially if you are running with an AMD CPU.
EDIT - did a quick search for you mobo and learned that it's a P4 board, but that's not a huge problem. It says 4X is the AGP slot (which you already said), and seems that 4X means 1.5V (I didn't know that before, and didn't really understand that
). You really shouldn't have to get a new motherboard for this, as it just means that you won't be able to run the card at an 8X setting (it will be stuck down at 4X).