Next month, NiGHTS into Dreams HD and Sonic Adventure 2 will be making their way to XBLA and PSN, as part of the "SEGA Heritage Collection" on console. You ought to find both of these classics individually priced at $9.99 or 800 Microsoft Points the first week of October.
For Xbox LIVE Arcade, the games will be released on October 5, while PSN gets the two titles a few days earlier on October 2.
Interestingly, NiGHTS HD will include "free access" to the 1996 holiday-themed Christmas Nights, a rare expansion disc formerly available only through the Saturn holiday bundle or other special promotions. Sonic Adventure 2, on the other hand, will have a "Battle Mode" DLC for $3.
While NiGHTS into Dreams HD was previously confirmed for Steam, the PC version wasn't at all mentioned in today's announcement. We can on assume that version is coming out later at this point, though we'll see if SEGA is willing to shed more light on the subject.