I have a Dell 4550 that has a AGP 2x/4x. If I go out and buy a video card that is 4x/8x, does that mean it only supports mobos that are 4x WITH 8x (4x/8x)? or does it mean that it supports 4x or 8x, meaning I can use it on my 2x/4x.
AGP is backwards compatible. So yes, an "8x" card will work in a 4x slot.
As for your Dell, I can confirm it myself as I used to have a Dell Dimension 4550 with a 4x AGP slot. It accepted my 8x Nvidia 6600GT as well as an ATI 9600XT (which also works in the 2x slot in my really old HP Pavilion).
btw - I'm not sure what card you have in mind, but if it's power hungry, know this. The psu on my Dell 4550 was only 250w, so I put in a 450w unit when I added the 6600gt (specs for which called for a minimum of a 350w power supply). I got lucky and my Dell motherboard didn't have a proprietary plug for the power, but I hear with many Dells, you won't be so lucky. I still had to cut a small hole in the back of the case to accommodate the new psu.
To be sure, check the Dell forums to see if what you are planning has been tried already and will work.