If the pre-amp you are buying is a cheap one and you are using a xenyx 802 personally I would not bother. I suspect you are talking about a ART tube MP, it does get good reviews but the difference would depend on the mic you are using as well.
However the other part of the question about the xenyx 802 pres after the art, you can run the Art output into a line input on the xenyx 802 and therefore not use the pre at all on the xenyx 802.
Cheers
Alan.