My current card has a Effective Memory Clock of 2000MHz while the one I'm thinking of getting has an Effective Memory Clock of 1100MHz (4.4Gbps). Does that mean the one I have is actually better?? Basically, I want to get a card that is slightly better than what I have now & also a lot quieter.
GDDR3 like on your GTS250 is "double pumped", meaning it can transfers 2 bits of info per clock cycle. So the "effective memory clock" of 2GHz means is actually running at 1GHz, which is normal for GDDR3.
GDDR5 on the other hand is "quad pumped", meaning it can transfer 4 bits of info per clock cycle. It's effective memory clock should be 4.4GHz, and actually running at 1.1GHz, which is normal for GDDR5. If where you're looking at has it written as EMC at 1100, it's probably a misprint.
The 6850 is faster then a GTS250. Its similar in performance to the GTX460. (which is like a GTX280/285.) It's a much faster card. Don't know about quieter as models are different depending on the cooler.