Does your computer get the job done as it is, without being excessively buggy or prone to crashing? If so, then I see no point in fixing something that isn't broken. To continually replace your machine based on some fixed time interval, with no thought toward whether or not you actually need to after that interval has elapsed, is just silly.

Regarding your graphics card, if that is primarily what you are interesting in improving then just replace the graphics card.

Save the bulk of that money for college life. I don't think anybody appreciably foresees how costly it can be.