
Biggest and most noticeable performance improvement for my bike is always a new rear mech cable.
Most noticeable difference also depends on what you're looking for!
Tyres, Handlebar grips & Saddle will make a BIG difference to how the bike actually 'feels', but some people would be much more satisfied from seeing pimpy brand names on handlebars, seatpost, etc.
I'm not convinced that XT kit actually has a noticeable performance difference to LX kit. Sure, it might be a fraction lighter or a touch stiffer, but I've never noticed any difference.
Beware of spending cash on bits that simply look pimpy.
If you're not using disc brakes, then I can assure you that they make a huge difference. Knowing you can stop is so reassuring. Hayes HFX9 are within your price range if you shop around, but not if you also need disc-ready hubs...
A new pair of shorts could also be a good investment! I just bought a pair of Endura MT500 shorts, and liked them so much that I went back and bought another pair the following week.
Actually, if you're not using SPD Shoes and pedals, then I'd say these should be upgrade #1