Difference in hashrate between two GPUs I am a gamer who simply uses his rig to mine when idle. A year ago I was getting 0.5 Ghs using a Radeon HD7970 3072MB 384bit OC Boost, then due to a technical issue I stopped mining for a few months and eventually replaced the Radeon with GeForce GTX770 4096MB 256bit OC...

On which I am currently getting a consistent 126Mhs.

What follows may be a dumb question, but 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 and BitMinter, GUIMiner, Win7
The general answer is that your card replacement caused the huge drop in hash-rate. Radeon cards are generally better for mining (faster and more efficient) compared to Nvidia. There is a nice chart that approximates hash rate per card here: https://litecoin.info/Mining_hardware_comparison

You should really consider mining scrypt algorithm at either a profitability pool (I think BTCGuild has one now) that will mine the most profitable coin and convert it to BTC for you, for a small fee of course. This should increase your mining yield by 10fold or so!

You also might try Cudaminer instead of GUIMiner for nvidia cards. I have had better using cudaminer for nvidia gpus than both cg and bfgminer.