stored procedures and dynamic queries

Any general rule about dynamically generated queries in stored
procedures vs. performances?

I was going to write stuff as simple as

create or replace function EditQty(int, int, int, varchar(10))
returns boolean as
'
declare
_uid alias for $1;
_aid alias for $2;
_qty alias for $3;
_table alias $4;
_modified timestamp;
begin
_table := 'shop_commerce_basket' || _table
_modified := now();
update _table
set qty=_qty,
modified=_modified
where uid=_uid and aid=_aid;
if not found then
insert into _table (uid, aid, qty)
values(_uid,_aid,_qty);
end if;
end;
' language plpgsql;

Is it going to perform worse than with a static table name?

Where can I find some clue about the effects of similar decisions?

thx

--
Ivan Sergio Borgonovo
http://www.webthatworks.it