I'd love to get into steam games! Most recent game I played was medieval 2 total war but it didn't run well at all (that's about 5 years ago, so a little more recent).
Yeah as others have said, wait for the Steam xmas sale for sure.
For the sale they just had last week, I picked up the Total War complete game pack, which had, Rome, Medieval II and Napoleon for only $12 or so...
Then I think I got Fallout 3 for like $5 and then new vegas was equally cheap or cheaper...
So if you are in "catch up mode" that's definitely the way to go... you can score some great titles that span the time range you've been away for dirt cheap, like $5 per game for example.
BF3 for sure destroys the console version, so that's one of the easiest games to point to as far as PC superiority is concerned...
What will your new PC look like? Probably want to make sure it can handle BF3 without any problems.
Oh have you played the Company of Heroes series? Those games are amazing, highly recommend them....
Obviously Half Life 2 if you haven't had the pleasure yet...
Mass Effect 1 and 2 is a given, Civ 4 or 5, or both