A PC hands down, there alot cheaper than a MAC, and while MACs are associated with design work the software used by such companies (usually the Adobe Suite, Dreamweaver mainly, and things such as autodesk products), will also have a windows counterpart.
My housemates on a web design course at university and the cheap £300 desktop i built her didnt even have a dedicated graphics card and it still handled all her work very well (and this is with the windows 7 aero themes still running, music, hd movies and such going on in the background so performance was no issue). In fact the only reason it now has a dedicated graphics (it didn't for 2 years) is because it came out of my rig when i upgraded.
So really £300 (minimum) on a pc or £700+ (last time i checked with a student discount) on a mac.
*EDIT* you'll also find alot of people still use the basic text editor to create there sites since it doesn't have features such as autocorrect.