Splitting a table for performance reasons

Hi,
I've got a table whose records are more less big. There's however jus one
Int-column changed frequently.
According to postgres' MVCC a whole record gets written, even if just one
bit was changed.

I think of splitting the table now in two parts, connected via the former
PK.

so like:
a|b|data | othercol | int     ->    a|b| data|othercol     +  a|int

Will this solve my ongoing performance problem?
Or is the overhead for referencial intergrity and system columns bigger than
what this gives?

BTW: How can I find out how many bytes a record consumes (having just fixed
size data)?