Maybe a 2009 or 2010 Rock Shox Recon solo air would be a good choice, about 3.9 lbs. Not as light as a Fox F100 but almost half the price at around $350.
Lighter tires like Kenda Kozmiks or Karmas, get 1.9's if you don't really need the 2.1's extra width.
Lighweight inner tubes shouldn't cost too much too and save about 60 grams or so a pair.
Lighter rims, spokes and hubs should come next when the budget permits. Losing weight on the wheelset is where you'll really feel the difference.
Handlebar, stem, seatpost, saddle and crank will also benefit from an upgrade if you want to loose more weight, I'm pretty sure the stock ones aren't too light.
Look for a handlebar that's below 190 grams, seat post and saddle should not be much more than 200 grams each and a stem should be about 140 grams or less. A Shimano 2010 Deore or SLX crank should be of decent weight and not be too expensive.
I'd stick with the 8 speed unless you really feel you need 9, just get an 11-32 or 11-34 cassette if you have steep climbs in your area. But remember an 11-32 or 34 will weigh more than your current 11-28. Switching to 9 speed will just give you more ratios but the first and last cog will more or less be the same.