Non-ECC RAM will give you a small performance increase (speed) over ECC RAM. For a desktop computer the extra data integrity check provided by ECC RAM is not usually required (RAM errors seem to be rare), however a server will have a large memory size and will most likely be sending many copies of the same web page(for instance) from memory across the net. In this case the heavy use and reliance upon RAM means that ECC is worthwhile. Therefore 99% of desktops do not use ECC RAM as it is of minimal use to them (and also probably to you too). So, get unbuffered non-registered RAM unless you own a server or do critical work on your desktop.
Hope that helps.
Andy