So easy to find a decent 400$ laptop. Best Buy just had 2 at 350$ that were pretty good. An HP and an Asus. 14 or 15" both with 3GB RAM Win7 Premium and I think a 320GB HDD. One had and AMD AthlonII X2 2.2GHz and the other was an Intel P6100 dual core 2GHz (current generation, same chip as i3's). In the 400$ range I see 2.5GHz AMD chips and around 500$ you get into the actual i series CPUs from intel starting at like 2.4GHz. The Intel CPUs are usually a little faster clock for clock compared to AMD and also get a little better battery life, but usually cost a bit more.
Pretty much stick with the Athlon 2/Turion2 (not the first generation) and the newest Intel i series chips or their derivitives (like the P6100). You can google the CPU model number if you dont know what it is and it should tell you if its one of those three. You should easily be able to get 3GB RAM and a 320GB HDD as well, even 4GB RAM and a 500GB drive. If you can, go for a 7200rpm hard drive over a slower 5400 rpm drive (not the end of the world either way though).
Or go browse some local adds from ofice depot, best buy, staples, frys etc. and anything that catches your eye post on here and we could say yay or nay. One advantage to shopping with local stores is you can go in and try the laptop and make sure the screen and keyboard work for you and the thing doesnt bend like a wet noodle.