For our latest CPU group test we've only included current-generation parts widely available at major retailers. But there are still several different platforms represented here, and whether you're building a new PC or upgrading an existing system, you need to make sure the processor you buy is compatible with your motherboard. This is largely determined by the CPU socket.
AMD
Of the AMD CPUs tested, all but one uses the latest Socket AM3 design with 941 pins, and all Socket AM3 processors are compatible with both AM3 motherboards and those using the older AM2+ socket. If your motherboard uses either socket type, you'll be able to choose an AM3 CPU. Some older AMD processors, however, won't work in Socket AM3 motherboards, so they're an option only for those upgrading on an older board.
Backward compatibility
If you're stuck with an old Socket AM2 motherboard (with no plus), you're unfortunately excluded from most of this month's CPUs, since Socket AM3 CPUs won't work that far back down the tech chain. Socket AM2+ processors can, however, be used. Note that AM2+ boards are unhelpfully labelled "AM2", so if you're not sure what type of board you have, check the manufacturer's website.
Good news about AM3
As for memory, DDR2 RAM modules will work with AM2+ boards, while AM3 motherboards are available with slots for either DDR2 or DDR3.
If you're thinking of investing in an AMD system, the good news is that Socket AM3 shows no signs of being eclipsed any time soon. The bad news is there's little innovation on the horizon, either: AMD's public roadmap for 2010 merely features incremental improvements to the existing Athlon II and Phenom II lines.
Conclusion
That isn't to say that AMD has nothing up its sleeve. The company's careful handling of the upgrade path from AM2 to AM3 bodes well for any future developments that may be announced. AM3 motherboards are currently quite cheap too, with a host of options available for less than $100, so a new system will cost you significantly less than some of Intel's latest innovations.
Intel
Most of Intel's mainstream parts still use the almost ubiquitous Socket LGA 775. It now covers the Celeron and Pentium ranges, as well as all Core 2 Duos and Quads, and if that's what you're upgrading to now you may not need a new motherboard; all of these CPUs support DDR2 memory, and some LGA 775 motherboards accept DDR3.
Recommended enthusiast choice: LGA 1366 and X58
The newer parts bring unnecessary confusion, however, so you'll need to be sure to pair your new CPU with the correct motherboard. All was fine when the first Core i7s arrived, with the i7-900 range introducing us to the X58 chipset and LGA 1366 socket. That's undoubtedly going to be Intel's enthusiast side of things, with boards currently starting at around $230. Along with the exorbitantly priced CPUs, a new system will set you back at least $1000, even before you add RAM and a hard disk. New models will only take that higher.
Recommended mainstream choice: Core i5 and LGA 1156 and P55
But the arrival of the Core i5 - along with the introduction of the LGA 1156 socket - is much better news for consumers. If you can get your head around the confusion of the lower-end Core i7-800 CPUs using this socket, rather than the LGA 1366 of their Core i7 siblings, this more mainstream platform looks to be the real juggernaut for 2010. These, along with the Core i5-750, arrived with the P55 chipset, and motherboards right now retail for a reasonable $130 and upwards. It's some way from the sheer affordability of a new AMD CPU and motherboard, but Intel had another trick up its sleeve.
Onboard graphics option - H-series
Sure enough, right at the deadline for this Labs, the Core i5-600 and Core i3 ranges arrived to take Intel down into AMD's price territory. These 32nm parts have the graphics processor on the CPU, so they don't require integrated graphics on the motherboard; new H-series chipsets were also introduced to support this. We were too early to get hold of final retail motherboard prices, but pre-sale listings were beginning to appear around the $150 mark. Expect prices to fall further after launch, as Intel looks to move its new architecture into true budget territory.
Conclusion
Put simply, buy an LGA 1156 board now and Intel's long-term approach means it should still serve you well when you next come to upgrade your processor. Just ask the legions who bought LGA 775 boards years ago and are only now seeing a need to move on.
You might also be interested in our graph showing price vs performance for current AMD and Intel CPUs.