There are (in my prediction/point of view) there will be a very small chance that the prices of 6850 drops in the launching of the 7770 (it needs time, up to 2 or 3 months). With your Processor, 6850 will somewhat not be balanced with your processor (Bottleneck). So you can get a 6770, then upgrade the whole of your system (or make a new build) with like 7850 or something .
with your current CPU and the AMD 6850, you are reaching bottle neck territory. i doubt you see any major difference between the 6770 & 6850 (not to mention a 7770). not to mention most 20" LCD monitors are 1600x900 res, easily handled by the AMD 6770.
i agree with the other about bottlenecking issue. 6770 be suffice for you but you still can take 6850 if you're willing to OC your cpu. btw what is your psu exact model? also i think it is better for you to add another 2GB of RAM for your system.
Intel core2 duo E6750 2.66 ghz.
2 gb ram.
20' inch led screen.
my budget is 100 pounds.($150)
My options are:
1) Asus ATI HD 6850 Direct CU V2
2)ATI HD 6770
3)Wait for ATI HD 7770.
I want to know that Is there going to be a decent performance difference between 6850 and 6770 on my machine?
AND how much price of 6850 will drop approx when HD 7770 will be launched?
My max budget is 100 pounds and the HD 6850 is available at this price at this moment.
With your CPU I'd say the 6770 is more then enough. I have a core two quad (2.4Ghz) and an old 8800GTX and my GPU still cap my CPU on many games. I know that as a fact since my brother had the same CPU with a 9600gts (weaker card) and upgrading the CPU for an i5-2500k keeping the same card, he can now play most games on bether graphix quality then me.
If your not willing to upgrade your PC for now go for the 6770 I'd say and save money to upgrade because you will need too soon.
P.S. 2Gb of ram may hurt you if your playing MMOs.