score:4
The Biblical evidence is a bit ambiguous in this particular case, but tradition definitely says that it was cross in the shape of a small t, the common one found in classical and medieval art. Some thoughts from the Catholic Encyclopedia:
It states the cross shape explicitly:
The cross on which Jesus Christ was nailed was of the kind known as immissa, which means that the vertical trunk extended a certain height above the transverse beam
First, Biblical proof that it could not have been an "X":
We gather as much from St. Matthew (27:37), where he tells us that the titulus, or inscription containing the cause of His death, was placed, "over", the head of Jesus Christ
And then confirmation that it was a "β" with historical evidences:
St. Irenæus (Adv. Haer., II, xxiv) says that the Cross had five extremities: two in its length, two in its breadth, and the fifth a projection (habitus) in the middle
Nonnus confirms the statement that Jesus Christ was crucified on a quadrilateral cross.
Interestingly, there seems to have been a seat for Christ as well (that fifth extremity).
St. Irenæus... says that the Cross had a fifth extremity, on which the Crucified One was seated
But, the foot support probably wasn't there:
[That] the Crucifixion there [was] a wooden support... is very doubtful.