which is better?

quad. basically it has 4 cores (against the duo's 2) that processes multi threads of information from your programs simultaneously.

From the one's listed above, "Intel Core 2 Quad Q6600(s775/2.4Ghz) is the one with better processing power.

Read below:-

Based on the revolutionary Intel® Core™ microarchitecture, the Intel® Core™2 Quad processor delivers four complete execution cores within a single processor, delivering unprecedented performance and responsiveness in multi-threaded and multi-tasking business and home use environments.

The Intel Core 2 Quad processor is at the center of today's most interactive and content rich software experiences. The Intel® P965 and 975X Express Chipsets support the Intel Core 2 Quad processor. This combination of processor and chipset is the ultimate engine for Intel® Viiv™ Technology-based PCs, offering an array of exciting capabilities and delivering an impressive level of performance for demanding users. IntelÂ�� Core™2 Quad Product Brief

Intel® Core™2 Quad Processor Features and Benefits

Intel® Core™2 Quad Processor Q6600

Processor Number Q6600

Architecture 65 nanometer technology

L2 Cache 8M

L3 Cache N/A

Clock Speed 2.40 GHz

Front Side Bus Speed 1066 MHz

Other Intel Technologies Intel® 64 ¹,

VT,

Execute Disable Bit ²,

EIST,

Quad-Core

Package FC-LGA

Chipset Intel® 975X Express Chipset,

Intel® P965 Express Chipset

Memory Type NA

Boards Intel® Desktop Board D975XBX

Slot/Socket Type LGA775

Pin Count 775-land

sSpec Number SL9UM.............

Intel

Core 2 Quad Q6600 (s775/2.4GHz)

That way you have 4 physical processors as apposed to only 2, ok the 2 cpu's are 3 ghz which is only 600 mhz faster but when you have 4 2.4 ghz cpu's able to do work , that leaves you with 2 spare cpu's on the same die to do extra work.

The Core 2 Quad of course.

Imagine 4 cores running with an 8mb Cache plus a Front Side Bus of 1066Mhz ... that's speed and power at the tip of your fingers.

quad is better for a reason

in the future applications and games will utilize more cores making the core 2 quad a much better purchase in the long run because the dual core would be faster in todays apps (due to its higher clock speed) but will lag behind the quad in the future.