Most emulators are freeware or open source; those would be legally permissible (since no copyrights have been broken). This would include things such as MAME, MESS, etc.
Dumps of game data and code (often called ROMs), however, if not homebrewn or otherwise in the PD/Freeware, are not. They are copyrighted, even if it has been 20 years since they came out, even if you can't buy them anymore anywhere, even if you think they should be free. They are still copyrighted, and hosting them would under no circumstance constitute fair use. No disclaimer will help you.
(Some companies have released some of their ancient games into the freeware realm, most have not, and some sell them on compilation disks for other platforms).
Keep it legal. If in doubt, ask a competent lawyer.