Plugging into the mic input of your computer even via a preamp is a poor method of recording guitar. THe sound card input is usually meant for recording the spoken voice for telephone quality conversations, you would be much better off getting an interface either USB if you are going two channels or less or firewire if you are considering more. tHis will link you to USB interfaces [[url=http://[/URL]="http://www.sweetwat…"]USB Audio Interfaces | Sweetwater.com[/]="http://www.sweetwat…"]USB Audio Interfaces | Sweetwater.com[/] you do want a unit that can handle instrument level input as well phantom power if you ever want to record using condenser mics in the future

Thank you that's what I needed to know

What are the benefits of having an interface rather than just a usb adapter cable that comes from effects pedals to the computer?

You asked "What is the best way to record a guitar directly to my laptop?" You can plug a preamp into the mic input, you can use a USB cable from an effects pedal. One of the most significant differences will be the preamps and the analog/digital converters and the quality of these items are what seperate $100 interfaces from $1000 ones. It all comes down to what you can live with as far as quality goes. Of course a lot of that will depend on what you are listening to it with. You probably won't notice huge differences on computer speakers even so called hi-fi ones, and even headphones can be deceiving and any method is only as good as the delivery system (amplifier) and the speakers.

On a cheap budget, something like a Line6 Guitarport.

A little higher, a Line6 POD into a decent USB interface - depends on Mac and PC which one to get.

Further up, an SM57, decent preamp, converters and interface.

Top of the line is immaterial at this stage I would hazard.

I'm using a Line 6 UX-1 on a four-year-old IBM Thinkpad/Windows XP machine and it's working just fine, both for guitar and voice with a Shure SM58 mic.
Tom