I also agree. I'd say we're at the last few years of Core 2 Duo machines being usable. Anything before them is just not worth keeping around. Not only are earlier machines slow, but are going to be pretty energy inefficient. Buying a newer system might actually save you money when you factor electricity costs.

For example, a 2009-2012 era Mac Pro draws more power in sleep than a modern Mac mini/macbook Air draws under full load.