SC2 is a lot worse for custom games and the gameplay is admittedly a little 'less' than the original SC, but it's still fun. RTW is a good game but also crucially obsolete, if you were offered Empire: Total War, then I would recommend that to trump all the other 3 games. Then again, neither your current system nor your new one will be able to run it.
Civ V is a good game. I have to say Civ IV was more fun, but it's one of the less complicated nation strategy games out there so I guess it's playable for you, but the large community in SC2 and general coolness of lobby-based blizzard custom games (As well as the kickass new skins) would make it my recommendation.