Not all Macs have had autosensing ethernet ports for that long-some have only had them for a couple of years.
If the main purpose is simply to share files between the two computers, Thinine's remark about routers is correct. However, since most people ALSO want to share an Internet connection, AND don't want to worry about manually assigning IP addresses, AND want to "just work" with their computers instead of fiddling around with more than is absolutely necessary, suggesting an inexpensive router as part of a simple networking solution is quite appropriate. You can find usable wired routers for as little as $30 if you shop around, and even better prices if your timing is right (I once bought a SpeedStream router for a little over $8 after a $20 rebate). Further, ANY router will work-there's no "Mac router" versus "PC router" issue involved, since almost all networking is standards based. If you're absolutely addicted to AppleTalk, you may be stuck (it's an Apple standard, but not an IEEE standard), but otherwise, everything you need for file sharing, network printing, Internet connection sharing, etc. is right there in an inexpensive box.