I'll take first punt at a recommendation and hopefully you'll get a few more suggestions. IMO it's better to go for the tele-zoom than add a x2 converter. My reasoning being you will effectively lose 2 stops so unless you have a fast lens (big money) in the first place you are going to be limited.

I usually only use Nikon or Sigma EX lens but I would recommend the Tokina AT-X AF 80-400mm 4.5-5.6 and I'm not saying that because I have one up in the equipment for sale thread It's not a top of the range lens and not the fastest auto-focus either, but the results are good. Search google and you'll find the usual variety of opinions on this one.

HTH Tony