C# 156 146 118 Chars
using System.Linq;class P{static void Main(string[]a){System.Console.Write(
a[0].Aggregate(0,(t,c)=>(t+c-64)*26)/26);}}
Ungolfed:
using System.Linq;
class P
{
static void Main(string[] a)
{
System.Console.Write(a[0]
.Aggregate(0, (t, c) => (t + c - 64) * 26) / 26);
}
}