I simply should concatenate my current block chain (3 files) into 1 huge file (almost 4.9 GB), name this "bootstrap.dat" and 0.7.* will load this without signature-checking into the bitcoin-client if I start it (and of course build a new blkindex.dat)?
It will load it, but only the first 193000 blocks will be done without signature checking.
Also, I think 0.7.x has a bug where no more than 2G of bootstrap.dat is loaded anyway. You can use -loadblock=file1 -loadblock=file2, ... though.