They have a MUCH greater throughput than traditional hard drives, and thus make better use of the bandwidth available in the SATA interface. So, a 64GB or 128GB SSD is a good option for your OS and your top pick of apps because they will load resources from the SSD faster than they would from a traditional HDD. I agree that the price per gigabyte of SSDs is outrageous and I don't recommend anything large, but since you are building a high-end system it would be wise to invest in a fast hard drive to make full use of the other components. Remember, it only takes one part to slow the entire rig down.