Laptop is a thing which needs many factors to become a best piece. Unlike desktop we have to consider performance,battery life,temperature,weight etc. Let us discuss them one by one. If you are looking for desktop configuration please go here.
Processor: Fast processor will give best performance but it will eat your battery as well. So a medium speed processor is more recommended. May be 2.0 to 2.4 Ghz , Go for AMD processor if you are a gamer.
Screen Resolution : Big screen means big power problem. If you are a long distance traveler don’t go for big screen size 12” or 14” is best than 15”+
Battery: 9 cell battery gives good performance but if price difference from manufacture is too much between 6 cell and 9 cell, don’t buy from there. You can go cheaper from market.
Graphics Card: Graphics card is a thing which you can’t modify later after purchase unlike in desktop,so if you really need it you should go for higher memory from purchase time only.
Hard disk : There will be always a confusion if you are getting it from Dell. Which one to buy 5200 rpm or 7200 rpm. In 5200 rpm you will be getting more space but you may think of performance difference,then let me clear you. Yep,7200 hard drives perform well but on other hand drawbacks are more. The hard disk eats damn battery power. It crashes often,you need to replace it. Moreover,it increases temperature of laptop very high. I do recommended to go with 5200 rpm one.
Keyboard with backlit or without backlit: However you believe you will not need backlit ever still believe me it’s a great thing and you will need it anywhere. Without backlit it’s hard to see keys in less light also. Go for backlit keyboard if you are getting it.
Wi-Fi: Grab a good card as much you can. Because it will harass you later if you go for a cheap card.
DVD Drive: You can have two type – Either Tray load or Slot load. If you go for slot load you will be able to load small size disks also which are non standard.
Please remind me if I am missing any main component, However Now a days you can get laptops at great prices that also from some very reputed companies like Lenovo. I am using dell studio 15 laptop, I am damn happy with performance, though price is very high from market starting range of laptops.