At this exact instant in time, if you're on a budget then LGA 775 is OK because we're only just starting to use the power of the i7/i5 and Phenom II BUT it won't last long and is a blind alley for improvement. Sooner or later you will have to purchase a whole new machine.

If budget isn't as much of an issue or you desire to spend in a new base for future improves, then bite the bullet now and construct a system based around LGA 1366.