Difference in hashrate between two GPUs
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.