I want to start out by going the fastest route before it gets too late.
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Also, is there a limit on how many scripts can access the same SQL or the same File.txt?

Concurrent access of a flat file

This in my opinion is always problematic. If you have multiple scripts reading the same file then there is no issue but if by concurrent access you are talking about a read-modify-write situation then the issues with flat files are legion. If you have a RDBMS available to you use it.

Databases concurrent access

How many concurrent accesses for database XYZ? The answer: it depends. Database scalablity is a mult-variable equation. Some of those variables have to do with the architecture of the database itself and others with the available bandwidth to the database instance, architecture of the hardware and other factors. The kinds of stuff that in my profession (Unix Engineer) cause me worry wrinkles and loss of sleep.

A rule of thumb answer for you: If you are talking dozens of concurrent users then probably MySQL will work for you just fine. If you're talking thousands then you might consider something more heavyweight. In fact you might even be talking about clustering at that point. Perl Monks works off of PerlCGI and MySQL I'm told and mostly handles the load just fine and you have many more than "dozens" of concurrent users on this site I'm sure.

From what you've stated in your post there really isn't enough information for me to give you an in-depth professional answer, but if I have a choice between using flat files and a database I'll go with the database every time.