Everything... I know you probably don't want to hear that, but that's a pretty outdated system. You could make an ultra budget build that will outperform that computer.
That system is pretty old, as the CPU uses socket LGA775. You might be able to get a Intel Core 2 Quad, which is still a very decent CPU. You also might want to upgrade to something like a GTX 750ti, but you will want to get a monitor with a higher resolution than that. The total cost of the parts without the monitor would be about $224.99, which is cheaper than getting a entirely new PC.