Where I live, three months of the year we need our central air conditioning. The remainder of the year we either need a small amount of heating, or a LOT of heating. Having my gaming machine add to the heat of the house is just a bonus. My TV providing 700W of heat? Bonus! Of course, the cost of electrical heat is higher than natural gas heat, but that heat pushed out from the computer reduces the total heat I need from natural gas. A bit of lost efficiency ($/but) yes, but you cannot consider it waste heat for most of the year.
Oh, and I upgrade on a 3 or 4 year cycle, so there is ongoing power efficiency gains, but there is absolutely no way that spending $1000s every 3 years will balance out the "savings" in lost electricity. In terms of effect on the environment, where I live the only fossil fuel used to generate electricity is natural gas, and that represents 15% of electrical generation.