I always start by looking at the latest chipsets. The motherboard is going to dictate what you can and cannot do with the entire system. Putting money in to a nice motherboard has many benefits. Think about the number of SATA ports, if you need error correcting memory, RAID and expansion ports. Also form factor. If you care about size then this is the logical place to start. From there you will now have a range of CPUs, and memory to choose from. I always buy the fastest speed memory that my motherboard can handle; this can make a huge difference in how "snappy" or responsive the computer will handle. From there you just build out to your needs within the specs of the motherboard.