Running 24x7, each watt will cost $1.00-1.50 / year for electricity priced at $0.11-0.17 / kWh, which covers most of the US. My desktop consumes about 200 watts at idle, and is six years old, so I would have spent $1200 on electricity if it had been running 24x7 since I bought it. If I lived in New York, it'd be more like $1800, and in Germany, more than $3000. Electricity cost definitely can matter. On the other hand, if you setup your computer to sleep automatically (or are really good about turning it off), you can cut the power bill by a factor of 3 at least. Power use in sleep mode is negligible.
Using the Anandtech link provided by ShintaiDK, you might save about 35 watts at idle (computers typically spend most of their time at idle). Let's assume your computer consumes about 110 watts now.
1. If you run 24x7 and upgrade, you'd save $35 a year in Ohio, $50 in New Jersey, $90 in Germany, and $110 in Hawaii.
2. On the other hand, if you "only" spend 6 hours a day on the computer and setup automatic sleep mode, without upgrading you'd save $80 in Ohio, $105 in NJ, $200 in Germany, and $250 in Hawaii.
3. Once sleep mode is enabled, upgrading would save a further $10, $12, $22, or $38 by location.
As you can see, under certain circumstances, it can make sense to upgrade, but in most cases, it's best to use sleep mode as much as possible and just consider power consumption next time.
As far as economic vs. environmental benefits go, others are right that replacing appliances, cars, and computers wastes energy, but sometimes the tradeoff is worth it. The cost of the energy in a product can only be a portion of it's cost, so if your energy savings outweigh the product cost, there's a good chance that the energy savings outweigh the embodied energy as well. Now coal used to make steel is a lot cheaper than gasoline used to power a car, so it's possible to save money without any environmental benefits, but it's still a decent rule of thumb.
Using the Anandtech link provided by ShintaiDK, you might save about 35 watts at idle (computers typically spend most of their time at idle). Let's assume your computer consumes about 110 watts now.
1. If you run 24x7 and upgrade, you'd save $35 a year in Ohio, $50 in New Jersey, $90 in Germany, and $110 in Hawaii.
2. On the other hand, if you "only" spend 6 hours a day on the computer and setup automatic sleep mode, without upgrading you'd save $80 in Ohio, $105 in NJ, $200 in Germany, and $250 in Hawaii.
3. Once sleep mode is enabled, upgrading would save a further $10, $12, $22, or $38 by location.
As you can see, under certain circumstances, it can make sense to upgrade, but in most cases, it's best to use sleep mode as much as possible and just consider power consumption next time.
As far as economic vs. environmental benefits go, others are right that replacing appliances, cars, and computers wastes energy, but sometimes the tradeoff is worth it. The cost of the energy in a product can only be a portion of it's cost, so if your energy savings outweigh the product cost, there's a good chance that the energy savings outweigh the embodied energy as well. Now coal used to make steel is a lot cheaper than gasoline used to power a car, so it's possible to save money without any environmental benefits, but it's still a decent rule of thumb.