...on which I am currently getting ~130Mhs.
What follows may be quite a dumb question: did anything change (eg some network property), that would explain a much lower hashrate on a better card? Or is there something wrong with my setup?
I'm using: BTCGuild/BitMinter, GUIMiner/BitMinter, Win7
The Answer is in your post. You changed cards the 7970 is MUCH more efficient at mining. Just because it is a "better" card for gaming doesn't mean it is for mining. You will be able to increase your hashrate some with cudaminer, but a 770 will NEVER compare to a Radeon 7970.
Also as mentioned video cards eat more power than they can produce mining SHA coins and you should look into scrypt type alt coins, even then if you are paying for electricity a maxed out 770 running at 200Kh/s+ will barely break even at .10 kWh