Invest in Small Business Act of 2008 - Amends the Internal Revenue Code to: (1) increase the exclusion from gross income of the gain from the sale or exchange of qualified small business stock from 50 to 100% of such gain and reduce the holding period for such stock from five to three years; (2) disqualify stock held by members of a 25% controlled group of corporations for such tax exclusion; (3) repeal gain from the sale or exchange of qualified small business stock as an item of tax preference for purposes of the alternative minimum tax; (4) repeal the maximum 28% tax rate on gain from the sale or exchange of qualified small business stock; and (5) increase to $100 million (adjusted for inflation after 2009) the aggregate asset limitation for determining eligibility of the stock of corporations for qualified small business stock tax treatment.