Never played Deus Ex, but about a year or two ago, I played Morrowind on PC and Knights of the Old Republic on 360. Morrowind felt really stiff and I couldn't get the hang of it anymore, but exploration was fun before and it still is. I just couldn't get into the combat. The graphics don't matter much to me. Knights of the Old Republic played just like it did before, except it was glitchy because of playing on the 360. Graphics were just as I remembered and everything was great (besides glitches).
As for older games, I played Legend of Mana and Thief: The Dark Project last year and both are very playable (except Thief being tough to work with in Windows 7). I think that all games could be still playable, you just don't have to expect everything you can find in a new game to be in an old one. They're different, and that's good.
Thief and Legend of Mana aren't that old, but they are to me--one came out when I was 8, and the other two years after.