which will be most beneficial?

-FSA SLK chainrings (current) or buy praxis chainrings? ($160)
-105 FD (128g) to buy Dura Ace FD (64g)-difference of 64g (can get for ~$90 on ebay)
-Look Keo sprints (250g) to buy Look keo carbon blade ti 16Nm (186g)-also a difference of 64g (have found in used but like new condition for under $160)

I'd go with upgrading pedals. You're dropping the weight from a more important place on the bike, and you'll feel better performance with upgraded pedals.

I personally ride the Keo Blades on my road and track bike for a year now and love them. No problems.

None - wait until the chainrings or pedals wear out, then change.

You can drop 44grms here on your pedals http://www.ebay.co.uk/itm/Titanium-Spin ... 4ab812759d" onclick="window.open(this.href);return false;

I would not switch to the blades . The seem to break quite a lot.

bought a DA FD. stumbled accross a great deal on CL last night. $45 for a brand new clamp on! I payed to have it shipped. 64g savings right there!