Because Core i5 750 is much faster than Q6600, and Core i7 920 is even faster than Core i5 750?
Jabbs, I'll post some data/score for comparison here so it would be easier for you to decide:
Handbrake H.264 Video Encoding
Q6600@stock 2.4GHz: 1,305
Q6600
[email protected]: 1,915
i5 750@stock 2.66GHz: 1,937
i5 750 OC@ 4.15GHz: 2,852
i7 920@stock 2.66GHz: 2,144
i7 920 OC@ 4.08GHz: 3,248
Cinebench R10
Q6600@stock 2.4GHz: 9,817
Q6600
[email protected]: 14,735
i5 750@stock 2.66GHz: 14,114
i5 750 OC@ 4.15GHz: 20,675
i7 920@stock 2.66GHz: 15,206
i7 920 OC@ 4.08GHz: 23,283
WPRIME 32M (in seconds)
Q6600@stock 2.4GHz: 18.047sec
Q6600
[email protected]: 11.734sec
i5 750@stock 2.66GHz: 11.796sec
i5 750 OC@ 4.15GHz: 8.004sec
i7 920@stock 2.66GHz: 9.125sec
i7 920 OC@ 4.08GHz: 5.991sec
Crysis (DX10, 64-BIT, HIGH, 1,680x1,050) with HD5870
Q6600@stock 2.4GHz: 12-35fps
Q6600
[email protected]: 24-45fps
i5 750@stock 2.66GHz: 20-50fps
i5 750 OC@ 4.15GHz: 33-55fps
i7 920 and OC: don't have data, but should be little higher than i5 750
I would not recommend dual-core CPU because games are now leening toward favouring quad-core over dual-core, as you can see the test result of
a 4870x2 running with a Phenom II 550 OC to 3.8GHz at dual-core vs quad-core (unlocked cores) here:
http://www.overclock.net/pc-games/659536-contagion-review-dual-core-vs-quad.html
You can clearly see that in Battlefield Bad Company 2...the Quad-core is about 50% higher in minimum and maximum frame rate, and about 100% higher in average frame rate.
Considering you are not a heavy encoder, Core i5 750 system upgrade would probably suit you the best and it cost just around £350 (CPU £150, Gigabyte motherboard that can overclock CPU to 4GHz £85, 4GB DDR3 1,600MHz RAM £85, CPU cooler £30).
If you don't mind paying more to be may be up to 10-20% faster, a Core i7 920 system upgrade would cost around £505 (CPU £220, Gigabyte motherboard that can overclock CPU to 4GHz £135, 6GB DDR3 1,600MHz RAM £120, CPU cooler £30). If you pay another £20-30 extra for the motherboard, you would have two USB 3.0 ports as extra.
While i7 920 would obviously fastest, it does cost £150-£180 more. While it may be about 10-15% faster than i5 750 in encoding, the increase in frame rate for games comparing to a i5 750 (if we are comparing them at the same clock speed) would most likely be minimal with probably 2-5fps in difference at most.
As for power consumption, Core i5 750 is lower than a Q6600.