Obstructions to realisation of dual finite spectra as suspension spectra Suppose $X$ is a finite dimensional CW-complex with top cell at dimmension $n$ and consider its S-dual denoted by $DX$. I wonder if there are any obstructions to find a space $Y$ and an interger $k\geqslant n$ so that 
$$\Sigma^kD(X)\simeq\Sigma^\infty Y_+ ?$$
For example, in the case of $X=S^m$ the answer is positive.
EDIT In the case of finite dimensional projective spaces, one may start from spaces $\mathbb{R}P_m^n$ and choosing $m$ and $n$ in accordance with James periodicity, we can actually compute such a $k$. 
The question is that is there any such $k$ for a given CW-complex of finite type (which of course after Neil's answer below I realise the answer is positive if one is to choose $k$ enough large, and obstructions are for specific $k$'s)
I would be very grateful for any references.
 A: Firstly, you say that the answer is negative for finite-dimensional projective spaces.  However, in this case, and for any finite complex $X$, the answer will be positive if we take $k$ sufficiently large.  Perhaps you are just thinking of the case $k=n$?  Anyway, I will assume that we have fixed some particular $k\geq n$ and we want to know whether $\Sigma^kDX$ is a suspension spectrum.
The most obvious point is that $H^*(\Sigma^kD(X);\mathbb{F}_p)$ needs to be an unstable module over the Steenrod algebra.  This is easy to check if you have a good understanding of $H^*(X;\mathbb{F}_p)$ as a Steenrod module.  Similarly, the module $M=K^0(\Sigma^kDX)$ has naturally defined Adams operations $\psi^q\colon M\to M[q^{-1}]$, and if $\Sigma^kDX$ is a suspension spectrum then these will lift in a compatible way to give operations $\psi^q\colon M \to M$.  If this does not settle the question then one can consider additive unstable operations in $BP$ theory or $MU$ theory.  These are harder to handle explicitly but the basic slogan is as follows: $MU^*(\Sigma^kDX)$ is functorial for isomorphisms of formal groups, and if $\Sigma^kDX$ is a suspension spectrum then this extends to give functoriality for all homomorphisms of formal groups. 
