At the top end of your budget @ 13" ASUS U36SD-DH51 13" Notebook $849
Core i5 2430M(2.40GHz) CPU, 4GB RAM, 640GB HDD 13.3" 1366x768 LCD w/ GT 520M graphics card.
3.7 lbs w/ 7hrs + on the 8cell battery. No DVD drive, speakers aren't great and only a 0.3mp webcam. 2yr factory warranty and 1yr accidental damage protection. ASUS U36SD notebook review
For 14" notebooks: HP ProBook 4430s $670
Core i5 2410M(2.30GHz) 4GB 500GB HDD 7200rpm 14" 1366x768 anti-glare matte LCD w/ HD Graphics 3000
4.51 lbs w/ 7+hrs on the battery in note-taking mode and included DVD drive.
This business class laptop best meets your 3 yrs longevity goal. They're a bit more rugged than the consumer class laptops. HP ProBook 4430s review (specs are better in the NewEgg model).
HP ENVY 14- $750 and my recommendation for an great all-rounder notebook.
Core i5 2410M(2.30GHz) 4GB RAM, 750GB HDD 7200rpm HDD, 14.5" 1366x768 LCD w/ Radeon HD 6630M graphics,
2yr warranty, best webcam and speakers. A bit short on battery life and also the heaviest @ 5.6lbs HP Envy 14 (2011) Review