help needed: experts only !!!!

500.00 deepest used front wheel that you can find that you're comfortable with.

100.00 Wheelcover for the rear

150.00 Race day only tires

40.00 Latex tubes

Then again, I'm no expert.  So take this with a grain of salt

I hear that industrial size tractors have very large bands that help spin the blades.

In all seriousness, though, I'd get a disc-cover for your rear wheel ($90), a deeper aero front wheel ($400 used), and a good trainer ($300 - Kurt Kinetic) for doing some hard interval work. The motor is what moves the bike and there's nothing better for building that motor than doing some hard 20' intervals at threshold! A trainer allows you to be consistent rather than being impacted by outdoor influences on a ride. Now, if you already have a trainer you could spend that extra $$$ on a good set of lower rolling-resistance tires and maybe a good pair of transition running shoes.

New wheels with good tires will be the biggest equipment boost and then as another poster mentioned, a good trainer. Always remember: Its all about the engine.

Assuming also that the $800 budget is above and beyond the normal maintenance budget; make sure your bike is in as good of condition (tune-up and fitted) as possible.

It's not "free" speed, but a powermeter (e.g. a powertap and wheelcover for racing) can be had for about that much used.  I say it's not "free" because you have to commit to using it correctly (it has a learning curve).  Just buying it and adding it to your bike won't make you any faster, but buying it an committing to training by it (and training hard), will probably be the biggest gain of anything.  It will also take you months to realize any gains, so the wheelcover, deep front and race tires/tubes is your best bet to be faster tomorrow.

http://www.competitivecyclist.com/product-accessories/2011-cycleops-power-tap-compdt-rr-465-complete-wheel-5905.351.0.html

and...

http://www.wheelbuilder.com/store/disc-covers/

The most bang for your buck is to train more. It doesn't cost much $, only an investment of time and effort. Have you maxed out the performance capabilities of your current equipment? Are you one of those people with the high dollar, latest and greatest stuff that gets passed by an old guy riding a 30 year old Schwinn?

Of course there's no telling what could benefit you as you have no recent logs or any race results.

Or maybe the legal definition of "expert" in a court- as in anyone who claims to be ($$$$)

Give me the $800 and I will my coaching advise............he is one example you can have for free.  Training smarter can be more beneficial than training harder.  More to come when I receive my check.