Genius Bar informed me that I need a new logic board for my 2008 Macbook. Is it worth getting the logic board on an older computer, or should I upgrade to a new computer?

I was having issues with my early 2008 Macbook not charging my battery. The power cord was fine, the battery was fine, the battery connector and power device were all fine. Turns out I need a new logic board. The computer is just over 4 years old and has been used rather heavily over 4 years of Undergraduate/Graduate study. Would it be worth the $300 or so to get the new logic board, with the possibility of other issues arising due to the age of the computer, or should I invest in a new computer at this point? And if I need to get a new computer, what would my 2008 model be worth?