Depends on your expectations. Currently I have a GTX-680 running in a system based on an ancient X2-6400+. (my second backup system) Figured I might as well throw it in since its not really worth selling and would otherwise be gathering dust.
Although I've never bothered to do extensive bench-marking, there's no doubt the CPU is a serious bottleneck even with just the 680. (which is much slower then a 970) Having said that, it is considerably faster in most games then the same system running a GTX-285 plus has better IQ. It also shows slight improvement over the same system running a GTX-570, although to be fair this may be due to improved drivers more then hardware.
Bottom line if you are replacing a low end GPU and are getting the 970 super-cheap or free then sure go for it. Otherwise I wouldn't even consider it unless you'll be building a new system very soon and this is just step one of the purchase.
What GPU do you have now? (if any)