Just keep an eye out for the Best Buy ad.
Every Sunday we have a $500-600 laptop. Typical stats for the $500 are Dual Core Pentium (Core based), 2gb ram, 160gb hard drive, 15.4" screen. Typical stats for the $600 are either: 15.4" screen, Core 2 Duo 2.0ghz 2mb L2, 3gb ram, 250gb hard drive, Intel 4500/3100 Integrated
or a 14.1" AMD Turion/Athlon X2, 3gb/4gb, 250gb ram.
Just make sure you walk in on Sunday or Monday and not a day after that or you'll be stuck having to pay for the "optimized" ones or they'll be sold out entirely.
A very good laptop can be had for $600 if you're willing to pay tax. Just this week, Best Buy has this:
http://www.bestbuy.com/site/olspage.jsp?skuId=9035572&type=product&id=1218010849257
Toshiba for $499 with Dual Core AMD, 3gb, 250gb hard drive, ATI integrated card, 15.4". It's ugly, but I promise you it's better than buying a "used Pentium 4 M" laptop on Craigslist.
If you don't want to go with those, get a Dell Outlet. Those are AWESOME. Check Slickdeals.net coupon forum for coupons on them. You can sometimes get as much as 30% off an outlet Studio or Inspiron. Typically the deal will be $500 can get you a Core 2 Duo, 320gb, Bluetooth, 3gb ram, wifi, webcam, any color you want. $600 can get you dedicated graphics card on a studio laptop.