When I build a system for someone else, I try to get the best parts that fits into their budget (best bang for the buck I guess). I try not to skrimp too much on any one area. Luckily, I haven't been asked to build a computer for $300 or anything like that yet.
For my own computers, I usually use leftover parts from other people's computers (I haven't built a brand new computer with all new parts for myself in 2 and a half years). I don't need power house computers for what I do, so using older technology works fine for me...that's why most of my own computers are based on the Intel BX chipset, Pentium II and III Slot 1 CPU's and Matrox video cards.
Cricket