Low power also means less heat.
Less heat would mean less cooling which can be quieter fans.
Also less AC is needed to cool your room.
Your power supply doesn't have to work as hard either, so it is more efficient and cooler.
You don't have to get a bigger power supply either. The Nvidia cards might be good but if you have to spend a hundred on a new PSU, it isn't really that good anymore.
I just replaced my house's AC and furnace fan just because my computer was being uncomfortably hot. Like really really hot. my computer uses over 600W. That is practically running a space heater all day long in the middle of summer. I never thought of power and heat when I bought my computer lol but now I know.
The HD 5770 is nearly as powerful as the older HD 4870; I'm talking about less than a 5% performance difference. The HD 5770 consumes around 63w off the top of my head and the HD 4870 is about 120w.
That's 57w savings per hour. Suppose you game 30 hours a week for a year. That works out to a difference 82.7KWH in 52 weeks. The national average per KWH is roughly $0.12, that works out to roughly $9.92 difference per year. If you live in a city where it's about $0.22 per KWH, then that's $18.19 difference per year. If you live in a rural area where it's $0.07 per KWH, then that's only $5.79 difference per year.