OKay LISA, actually sata 150 is regular sata.
Sata 150 = "sata 1" 150 mb/s
Sata 300 = "sata 2" 300 mb/s
...there will be no performance difference running your sata drive in IDE mode(the controller on the motherboard controls the MODE) as opposed to sata mode.
Furthermore if you switch to sata mode in windows xp and did not install the sata drivers at setup using the f6 key, or slipstream them, windows will crash when it boots, or reboot continuously.
If you have vista, then it will boot fine, but still make no performance difference. Your asking for a lot of probable hassle for no gain.