Well depending on your operating system sense alot of the really great older games are just written to play on any Win OS up to windows xp 32bit OS some dont work on XP Pro 64 bit
32 bit OS Games, All of the Call of Duty Games the offline storyline missions are great, they online action is fun to , Nova logic games I.E Landwarrior series, Delta Force series, There's a couple of good Air type games also by Nova Logic. Battlefield 2 , All of the Medal Of Honor Series, i guess it just depends on what you enjoy playing..
My system now just wont play all of my older 32bit OS games, so i still keep a PC with xp 32 on it, to play them. Games im playing now on my 64Bit and really like are Modern Warfare 1 and 2 , Resident Evil, L4D, trying the L4D2 Demo ( lol fun) Fallout 1, 2 and of course AH2