First up make sure what you want to built before going any further. Though it sounds oblivious, but most of the times the builder has only a rough concept of what features they want in the build thinking everything will fall into place at the right moment. But actually, that right moment tends to come only after one or more components are came back due to incompatible with the rest of the system. Common problems are:

• Underpowered Power Supply Units (PSUs): the wattage is not compatible with the current capacity of the individual voltage rails.

• Wrong or mismatched interfaces: a PCI-e video card will not support an AGP slot and vice-versa. Same as, a 939-pin motherboard (Athlon) can’t be expected to have a Pentium IV processor (427 pins).

• One component too fast or slow: why purchase a memory designed for a 400MHz front-side bus if your motherboard can only maximize up to 333MHz? If you want to overclock something, then you are required to know now only, so that you can consider it while buying.

Make a note of down all the requirements for the build, satisfying maximum details possible. Then find the parts fulfilling those requirements. Confirm that all the components complement with each other before buying.

Apart from that, knowing precisely what is required will help prevent the unwanted emergency addition. Finding out that you require a strong or varied style of heatsink for the processor is better as while attempting to assemble it not only delays the build, but also makes it more expensive.