How do HDD cards affect PCI bandwidth

Hi

I have a mobo that only works at UATA 66. I am short on storage space and am thinking of getting 120Gb ATA100 hard drive. While i am upgrading i was also considering a controller card (about ?20) so that i can use the hard drive at the full ATA100. How will the card affect my PCI bandwidth and access to graphics cards in relation to games/windows/system stability?