Mode S XP will be mandatory for IFR early next year.
Mode S XP is proposed to be mandatory for everything that flies in 2008, but nobody yet knows if there will be exemptions for some categories of planes e.g. those without an electrical system, non-CofA planes, or whatever...
Personally I would buy a used Mode C XP. It will trigger the TCAS of any nearby airliner (not that you, with your top-end GPS, will ever stray into controlled airspace
) and any radar unit will see your altitude readout which (even though "unverified" if you aren't talking to them) is of GREAT benefit to everybody else who is getting a radar information service.
Then, if you need Mode S in 4 years' time you can pull out the Mode C and put in a Mode S, little or no installation cost involved. And they will be cheaper by then.