check to see what the best cpu your motherboard can support. then hit the ebay or buy new. adding more ram isn't going to help you much in terms of LFS so i would not waste money on that.
in your case, a small upgrade could make a big diference.
Doesn't matter do I buy i7 or i100. That's just the truth that graphics card is as well very old. In two years I'm planning to buy new computer, when I will finish my studies and start live alone..
Antialiasing is supposed to smooth the edges of objects so that they don't look so "pixelated". I think that this image demonstrates pretty well what does the AA do.