How much DNA of mitochondrial origin is incorporated in main cell's DNA?
Interesting question. I researched this a bit now and the phenomenon is termed "numt" for  "nuclear mitochondrial DNA". This term descrives the transfer of cytoplasmic mitochondrial DNA sequences into the separate nuclear genome of a eukaryotic organism. It seems that most of these sequences are inactive. This list at pseudogene.net has a large number of sequences in a lot of different organisms, for humans insertion between 0,2 and 14,2 kb are listed. Most of these sequences are detected in the deep sequencing projects underway these days and by comparative sequence analysis. An example is this paper, where they found mtDNA on chromosome 1 of homo sapiens, which was still part of the mitochondria in homo neanderthalensis.
For more information you can go to the list of papers I list below, this should be a good starting point:


http://www.ncbi.nlm.nih.gov/pubmed/11369110
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020273
http://www.ncbi.nlm.nih.gov/pubmed/15014143
http://www.ncbi.nlm.nih.gov/pubmed/18721785


The papers should all be accessible, if not, let me know.