Windows 7 install questions.

So i downloaded and burned the windows 7 RC build 7100 iso and preparing to install. Im currently running vista home premium 64bit and im about to drop kick it in the teeth (figuratively) b/c of sluggish performance with high end hardware. Im going to install and use win 7 as my primary OS from here on out and here are my questions:

1. Should i reformat my HDD for a clean install? Be nice not to have to back up files and music but i dont want to risk some sort of error that could comprimise the files. Or should i install it on top of Vista?

2. Sounds noobish since im a hardware junkie, but the one thing im not sure about is getting my sata drive up to its 3.0 gb/s standard speed from its current Ultra DMA5 setting. In my bios, i have 3 settings to set my sata drive to: IDE (current), RAID, and ACHI. which of those three do i set it to?

any info will be appreciated.