I am afraid I have some bad news for you. The reason your Dell Dimension 8300 is in IDE mode is because there is no ACHI mode. The 8300 was first released in May, 2003. That is 3 years before Tom's Hardware published the very first ssd reviews. Your motherboard will not support a modern 3rd generation SATA 3 6Gb/s ssd. There is no fix for it.
i7Baby - The motherboard is 11 years old so there is no valid reason to post it is another reason not to buy a Dell. Actually consumer ssd's did not become popular until 2008 - a full 5 years after the motherboard was released.