Review Today, Intel's officially released Pentium CPUs that offer both high clock speeds and dual-core loveliness, although you won't get both in one package. The Pentium 4 660, which is a 3.6GHz 'Prescott' chip with 2MB of L2 cache, will now be play second fiddle to the Pentium 4 670. All the same internals; just 200MHz faster and, obviously, more expensive. On the other hand, the near-£700 3.2GHz HyperThreading-capable Pentium Extreme Edition 840 gets a little brother. The Pentium D 820 runs in at 2.8GHz, is dual-core, but does not support HyperThreading. The end result is a dualie that comes in at a more palatable £200 or so. Which is better: high clock speed and HT (the 670) or relatively low MHz and two cores (the 830)? Are either of them worth it?

The 500-series P4s carried 1MB of L2 cache, an area of on-chip memory that stores data for the thread currently being executed. Running at full core speed, accesses to L2 cache are orders of magnitude faster than getting data from the system memory. Generally-speaking, the larger the cache, the better, though after a point adding more makes increasingly little difference. Gaming, in particular, sees a boost with larger on-chip caches. The Pentium 4 670 has 2MB of L2. Adding an extra 1MB of cache pushes up the 600-series' transistor count to around 169m, around 35m more than a 500-series part. The cost is inevitably passed on to the consumer. A Pentium 4 560J weighs in at around £270. The equivalently clocked 660 hits the wallet for £380.

The P4 670

All 600-series P4s support what Intel terms C1E Halt State. Put simply, with an appropriate OS and motherboard BIOS to activate it, C1E drops the CPU's multiplier and voltage to lower levels when a HLT (halt) command is issued. It makes implicit sense; why run at full speed and voltage when the system is just ticking over? Once an application demands CPU power, the voltage and multiplier are raised back to performance levels. There's also Enhanced SpeedStep (EIST), which is similar to C1E, and Thermal Monitoring 2, which intelligently reduces clock speed and voltage if the CPU begins to overheat.

The 600-series also benefit from Intel's AMD-like 64-bit extensions that allow the CPUs to run 64-bit operatings systems and applications. Windows XP 64-bit has already shipped and drivers are slowly but surely getter better. NX Bit, when implemented, stops code from being run in certain areas. It looks great on paper, but the majority of naughty viruses use more creative ways of execution.

The 670 is still stuck on a 200MHz frontside bus. It's strange that Intel chipsets have supported a 266MHz FSB for some time now and DDR 2 SDRAM is hitting the 400MHz mark with ease, yet all P4 processors barring Extreme Editions run at 200MHz. A 3.73GHz (14 x 266MHz FSB) model would have been appreciated.

The 670 has a TDP of around 115W. Cooled by an Intel reference copper-bottomed design, the processor hit 66°C running. It's one toasty CPU.