GMAT Help (reviews)?

I am studying for GMAT as of today. Here’s what I’m planning on doing studying 12 hrs a week for 8 weeks.

1) I have the princeton official GMAT guide 2007. does 1 year really make a difference?

2) I am planning on buying the Manhattan GMAT guides, but I’m not sure what to purchase, can someone help me figure out what I would need? I’ve heard these guys set the standards.

3) Going to take atleast 4-5 practice GMAT test before I take the actual one, any guidance on where I can find free practice tests? If not I’m willing to purchase for a small amount that explains the answers.

Any help would be greatly appreciated.

Thanks in advance!