The language you use would really have more of a bearing on the type of software you install on whatever machine you get. And the features of any specific machine would depend on how heavily you plan on using it and what you will store on it. For example if you were to design websites that were image and video intensive, you may need a huge hard drive or to buy an extra external hard drive so you don’t run out of space. That’s just one example.
I would say you would be ok just buying something in 4-600 dollar range with a couple hundred GB hard drive, 4GB of RAM and a current processor. I’ve had good experience with the intel core i5 processors running things like Adobe Premiere Pro and Photoshop.
I would steer clear of netbooks and get something that has multiple USB ports and a disk drive (CD).
Really you could build a website on any machine but you want to be sure you have tools and hardware that will make the process smoother for you.
Hope that helps,