With a HTPC, some of the most desired traits include low power consumption, quiet operation, all while maintaining a high performance level so you can do things like transcode video quickly. Older components that you have outgrown don't tend to be nearly as efficient as newer components. To have a good HTPC experience, you really want to pick components from the ground up, which is why I was excited to take a look at the Steiger Dynamics Maven Core HTPC.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips