1. The perfomance difference theoretically is supposed to be double .. but practically i dont know if its dat much but there will surely be some diff between the perfomance.
2. SATA II is backward compatible with SATA so it will work on a SATA supporting mobo at SATA speeds.
3. All Nforce 4 chipsets support SATA 2 .. good ones are ASUS A8N-E , ASUS A8N-SLI PREMIUM , MSI K8NEO4 PLATINUM etc.
4. I suggest going for a SATA II HDD from Hitachi as their prices are almost the same as SATA and they come with a 5 year warranty .. there is no sense in buying SATA now when SATA II is available and that too at the same price.