If they are the same price, yeah! Assuming it will fit in your case and you have a sufficient power supply of 500-600 watts or more. The 5770 is a larger card that takes up more space and need additional power input most likely. But it's a slightly better card. The 5670 is pretty good though. Nothing to complain about there.
...a lot of the new 5770 are built on smaller boards similar in size to the 5670 and the card does not need a 600w PSU unless you bought it from walmart. A solid 400w unit is more then enough for the 5770.
When I was building my own system, this thought never occurred to me. Well all the people already answered the question, for $40 extra there is a huge performance jump and both cards doesn't support Eye-finity? That feature is only useful if you plan on having annoying breaks on your screen because your linking multiple monitors together. I say it is well worth the upgrade and if you plan on CrossfireX that card basically beats out 90% of any single gpu on the market maybe even every single gpu card on the market lol.