Dont buy a MAC, not because they're not good, just because they have a reputation for side-stepping thier customers....MAC's a relatively expensive for the customer service you get.
What you need, depends on what you want to do with your laptop. Are you editing digital photos, digital movies, music? Do you just want to surf the net? Are you a gamer?
With today's technology you should expect to have the following for a decent all around experience :
Processor : 2.5Ghz or better, 1.8Ghz or better if dual-core
RAM : at least 1GB (512MB is skimping)
Video Memory : 256MB minimum if you plan on doing ANY video editing or gaming (512MB would be ideal)
Hard Drive : this can vary depending on what you want to hold, 80-120GB is enough for the average user who is just using the PC to surf the net, word process, and store photos and music. but if you are using your PC to edit videos and music and the like, you probably want at least 200GB
Also with a modern laptop, you should expect to get an Internal WLAN Card, Bluetooth would be nice, and a built-in webcam is nice as well.
Average retail price is about $699 for a decent laptop, but if you search hard enough you can find it for about $500-550. And if your willing to take a refurb, check geeks.com