From what I remember your board should have AMI bios, in which case one long beep would mean one of two things, there was a problem with your graphics card or with your ram, by putting a little pressure on them your most likely reseated them causing the problem to be fixed.
The graphical artifacts that you mentioned lead me to believe that the problem lied with your graphics card. I have experience such artifacts after having a graphics card partially die on me.
I would not worry to much about it unless you start having other issues with booting up the computer in the next couple of days.