X-bit Labs recently compared five mainstream processors: four Core 2 Quads from Intel, and one Phenom X4 from AMD.
Phenom X4 9950 Specs:
- Codename: Agena
- 2.6GHz Clock Frequency
- 2000MHZ Bus Frequency
- 4 x [64KB +64KB] L1 cache
- 4 x 512KB L2 cache
- 2MB L3 cache
- AM2+ socket
- 65nm production process
- 125/140 W TDP
- Retail price: $174
Core 2 Quad Q6600 Specs:
- Codename: Kentsfield
- 2.4GHz Clock Frequency
- 1067MHZ Bus Frequency
- 4 x [32KB +32KB] L1 cache
- 4 x 4MB L2 cache
- no L3 cache
- LGA775 socket
- 65nm production process
- 95 W TDP
- Retail price: $183
Core 2 Quad Q8200 Specs:
- Codename: Yorkfield
- 2.33GHz Clock Frequency
- 1333MHZ Bus Frequency
- 4 x [32KB +32KB] L1 cache
- 4 x 2MB L2 cache
- no L3 cache
- LGA775 socket
- 45nm production process
- 95 W TDP
- Retail price: $193
Core 2 Quad Q9300 Specs:
- Codename: Yorkfield
- 2.5GHz Clock Frequency
- 1333MHZ Bus Frequency
- 4 x [32KB +32KB] L1 cache
- 4 x 3MB L2 cache
- no L3 cache
- LGA775 socket
- 45nm production process
- 95 W TDP
- Retail price: $266
Core 2 Quad Q9400 Specs:
- Codename: Yorkfield
- 2.66GHz Clock Frequency
- 1333MHZ Bus Frequency
- 4 x [32KB +32KB] L1 cache
- 4 x 3MB L2 cache
- no L3 cache
- LGA775 socket
- 45nm production process
- 95 W TDP
- Retail price: $266
"One of the best choices among low-cost quad-core processors is still the old 65nm Core 2 Quad Q6600. Intel has dropped its price so significantly lately that is remains in the spotlight despite its age and relatively high power consumption. Especially since in some applications such as 3D games, for instance, it manages to perform as fast as a more expensive Core 2 Quad Q9300 thanks to large L2 cache. Its overclocking potential is also pretty encouraging. It doesn't require a specific mainboard: you can almost always push its frequency 1.5 times up with just an efficient cooler." [X-bit Labs | Comparative Tests of Inexpensive Quad-Core Processors]
"... the only drawback of Core 2 Quad Q6600 is its relatively high heat dissipation and power consumption, which makes it unfit for quiet and low-power systems. The new Core 2 Quad Q8200 will suit much better for them. Despite much smaller L2 cache, this CPU is pretty fast and outperforms the fastest processor of Intel's competitor - AMD Phenom X4 9950. As a result, Phenom X4 family may be of interest only to those users who want to get a quad-core processor real cheap, but are ready to put up with low overclocking potential, low performance and high power consumption." [X-bit Labs]
AMD fans don't have much of a choice right now, having only the Phenom X4 9950 (or the 2.2GHz X4 9550) as their choices. Intel fans, however, can choose between the Core 2 Quad Q6600 (good performance and overclocking potential) or the Q8200 (for low heat and power consumption).
Recently announced were 3 new Phenom X4 CPUs: 
