Well, there's been 5 GTA games for the PC, so you've got plenty of catching up to do if you've only just played the first one... :lol:
I assume you've just finished "San Andreas"? Why not give vice City or GTA3 a go? The graphics aren't quite as good, but the games are still a lot of fun. I personally preferred GTA3 the most, but that may be because it was my first GTA game...