First let me thank you for your help,
Basically it would work to read each line. But, if the file is VERY large, then it will lterally never work .
The problem with the buffered string was, that it does not compute the HashValue of the whole file. I need every byte in the calculation. Well now I came to the conclusion I could take a Hash from each line, and not adding every line into one variable.
Then I could add the checksums of every line, which schould be far smaller and create a checksum of the single ones .
This is a pretty difficult thing i guess.
Maybe it would be possible to use

MD5CryptoServiceProvider().ComputeHash(ASCIIEncoding.ASCII.GetBytes(args));

with a filename? *g*

I got the idiea for the program from one called RetroSFV, it can create checksums for 1gig files within 30secs. I guess it's written in c++.

I would be happy for further support, but you've probably already heated your heads too much with this