"That's what I'm talking about"
I would translate it something like:
"To jest to o czym mówię" - It's not an idiom, just regular sentence - more literal translation
or..
"To jest to!" - and it's more like an idiomatic phrase - also very close to the English term from above.
"If I do say so myself"
You can't translate it literally. And if I understand it correctly, the below are different Polish version which may more, less fit this phrase..
"Jeśli mogę tak powiedzieć" ("powiedzieć" may be also replaced by "rzec") = If can I say so
"Jeśli wolno mi się tak wyrazić" = If I can say so
"Że tak powiem"
"To jest tylko moje zdanie" = It's just my opinion
"Moim zdaniem" = In my opinion
My English knowledge is limited so maybe someone else choose the most suitable translation.