The ageing nature of AMD's CPU architecture has its downsides, but there are also benefits for anyone hoping to build a decent rig on a tight budget. The AMD Athlon II X4 645 is a quad-core chip based on the finest processor cores AMD can currently muster, despite being a budget CPU.

In other words, we're talking four 45nm cores complimented by AMD's on-die memory controller and speedy HyperTransport interconnect. Putting clockspeed to one side, the only architectural advantage enjoyed by AMD's Phenom II CPUs is a dollop of shared L3 cache memory. The Athlon II models must survive with 512KB L2 per core. Still, the 645 has a healthy stock clockspeed of 3.1GHz, helping to offset the missing L3 cache.

The comparison with Intel processors, of course, is another matter. The Core i5 quad-core CPU may not be the most advanced Intel has to offer, but it's still a much more modern architecture.

In thread-heavy applications, the 645 puts on a good show. We reckon few people will really notice that their CPU completes a 3D rendering job in one minute and 16 seconds rather than the minute flat of the more powerful quads from AMD and Intel. Likewise, 17 frames per second compared to 20 frames per second when video encoding is surely close enough.

Where the performance gap is more critical is gaming. A few extra frames per second here can really make the difference between silky smooth and jerky junk. The 645 is well behind the Phenom II X4 970, much less Intel's lightning quick Core i5-760. Admittedly, the 645's 50fps average looks okay, but the occasional choppiness in our World of Conflict benchmark is unmistakable.

We liked

A four-core chip for chump change is an attractive idea. The Athlon II X4 645 isn't quite as cheap as we'd like, but it's still extremely affordable and delivers excellent threading performance for the money. That's thanks in no small part to its healthy 3.1GHz clockspeed.

We disliked

In the broader context of the cost of building a PC, £40 to £50 is not a huge sum. With that in mind, we'd be tempted to pass over the Athlon II X4 645 and spend a bit more on one of AMD's full-power Phenom II chips. That's especially true if gaming performance is a high priority. The cut-down cache hurts the 645's framerates.

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