The Dell 4550s use the i845PE which uses the ICH2, ATA100 only, no ATA133. As said earlier by another poster, ATA133 is nice but it isn't going to impact performance much if at all. I personally tend to like to use ide drive controllers embedded in the chipset rather than running off the PCI bus.
So andrewchen46, an ATA133 drive running on a Dell 4550 will run at ATA100 mode will little or no performance loss.
theanimala1, I'd sell the card on ebay, not sure how much you would get tho. Remember to mention the brand of the card.