Hello,

to 1.:

No OS can "Destroy" a HDD, in most cases When u format a HDD to HFS u can simple delete the Partion and format it in NTFS or whatever u want.

So, should the OSX installation fails, u can boot from your Vista CD and install Vista, deleting your crap´t OSX. Just so simple

to 2.: I dont have a ATI, but i see kext for the 4600 HD card. So i would say Yes, it will work.

to 3.: Dont make it so complicate Install Chamalion Bootloader, and u can boot from as many OS as u like.

I can recommend IdenebV1.4 10.5.6 a great and very Simple Distro. Or IPC 10.5.6
But for a beginner i think Ideneb is a bit easier.