I would invest in: 1. web 2.0 properties 2. article marketing 3. guest blogging (blog post / writer's cost) 4. microsites
blogroll and getting edu links , edu links are nofollow but great for making your is look natural and concentrate on getting only pr4+ sites
Few examples of web 2.0 properties: 1. Squidoo lenses 2. Hub pages 3. Ehows 4. Wiki how 5. wordpress blogs 6. blogger sites and so on...