"The psalm is an integral part of the liturgy of the word and is ordinarily taken from the lectionary since these texts are directly related to, and depend upon, the respective readings".   If you take all three cycles of Sunday Mass and add in the main special feasts and celebrations you will find that there are almost 200 Masses - and almost as many specially selected psalms with specific responses.   This places quite a burden on musicians and assembly alike unless you happen to have a reasonable collection of confident cantors.   With that in mind it was decided that there should be a collection of psalms known as common psalms which can be used instead of the psalm of the day in particular seasons.   Please discuss any changes to the psalm selection with other interested ministers - it would be a bit confusing if the homily and/or bidding prayers made reference to a text that wasn't there!   For more on this take a look at Tony's introduction to the responsorial psalm.

Gospel acclamation - this is an alleluia for most of the year but is replaced during Lent, when we save it up for Holy Thursday and Eastertide.   There are plenty of beautiful Gospel acclamations for Lent so don't panic.   The special verses printed in the lectionary, once again, tie in with the readings of the day - if you can find a way of singing the specific verse - perhaps the choir or a cantor singing to a simple chant (that's what we often do) then you will be adding further relevance to the acclamation.   Technically the "... alleluia or the verse before the gospel may be omitted if not sung..." but, having said that, I feel that it is a wonderful sign of unity of the assembly as we greet the Good News.