The IBM 5170 PC/AT was the third generation in the PC series released in 1984 and introduced a new processor to the market -- the 286. Offered in 6 and 8mhz speeds, it still doesn't make them much of a scorcher, but with a 16/24 bit processor, 16-bit ISA bus, Power-supply and HDD lights mounted on the front console and 1.2mb Floppy drive it sure as hell sounds alot more modern. The reason I call it the cross-over machine is because the first generation of the PS/2 models had specifications similar to the AT and used alot of the same hardware.
I have never used one of these although I am considering buying one to run alongside the XT, I feel that they must be at least 2 to 3 times faster than an XT. A 286 processor itself is faster, and the extra clock speed gets things done quicker. I also don't know if these machines use the older socket from the 8088 processor or the more modern one used for the 10mhz + 286/386/486 processors. I have a 286-10 processor lying around with nothing to plug it into, so I know it uses the gold pins whereas a 8088 used silver colored pins much like you would see on an IC chip.
A well-equipped machine if you got one of the more expensive models. The basic model alone was over $6000, go for the top of the line and you were paying close to $9000. Can you imagine paying 9 grand for a PC today. And this was back in 1984! The $50 dollars they are selling the AT for at a local computer store doesn't sound all that bad right now.
The AT model also marked the last time that IBM would impose a standard to the PC compatible industry successfully. IBM attempted to impose the PS/2 Micro Channel bus but since they did not release the specifications to card makers and demanded an outrageous license fee for each computer sold, other companies like Compaq, Packard-Bell and many other large computer makers decided to create their own new bus standard, EISA. So when production of the AT ceased, so ends the compatibility with modern clone pc's until IBM ceased production of the PS/2 models in the early 90's and adopted the standards imposed by the other computer makers.