Performance wise the 800 card is more powerful see chart http://www.tomshardware.com/reviews/geforce-310-5970,24... The X800 cards only supported shader modul 2.0 which was fixed with the X1xxx generation supporting 3.0. The only reason to swap out the cards is if he is trying to play a game that requires shader 3.0 support.
X800 is faster, but of course it only supports SM2.0/2.0b not 3.0. The 2400 supports 3.0 so it will play games the X800 can't (technically), but in SM2.0 it's slower than the X800. As for DX10, the 2400 isn't really playable in any DX10 game.