For the current set there to be replaced by a new board, cpu, video card, cpu cooler if running stock hsf now, and certainly memory prices have come down or are lower on certain things. You can look over the estimate for a mid ranged upgrade with nothing elaborate.
Board(micro atx AM2?) $70-$110
cpu(AMD or Intel) $150-$190
mid range video card = $110-$140
memory = DDR2 533(slow), 667, 800 AM2+ 800 and faster 2gb $100-$140
sound card optional = $80 Creative XtremeGamer or Diamond model