Do your prospective opponents usually play Type 2 decks? Then I'd say don't waste your money on the stuff from Urza's block, since it'll be rotating out in a couple months, and instead focus on Mercadian Masques, and the new set, Invasion, coming to a store near you in October.
OTOH, if format doesn't matter and you'd like to limit your spending a bit, Urza's or Tempest blocks would be perfect for you, since those sets all had some really powerful cards in them, Urza's maybe a little too powerful, if you ask me. Alliances was great too, though I doubt you'll find very much of it around.
Not to steal Spiderman's idea, but there's a thread in the Issues Forum where we talked quite a bit about the different expansion sets, and it was a voting topic in case you're interested. Maybe by reviewing those you might get some ideas about the relative strengths of the sets, and base your decisions on that.
Welcome back to the game!
