well whenever you consider a dual system, its going to cost some money, even building your own. of course building it will save you some money.
you could get 2 opteron 242s or 2 xeon 2.4ghz cpus for the same price, so its more about what you want to use a dual system for. either of those you can buy 2 for ~$400 , then of course pick the rest of the parts that suit your system. you can definitely get more out of your money if you build it yourself. for oen thing, its wise to have at least 512mb of ram per cpu, 1gb per cpu would be best since you can use twin 512 dimms per cpu, taking advantage of dual channel supported by both opterons and xeons.