From the April 2008 Tri Mag:
100 representing the best value in terms of time saved per dollar spent
Tight fitting Clothing 100
Low rolling resistant and aero Tires 80
Aero Helmet 46
Training Tools (HR Monitor, coaching, power meter, etc.) 18
Spending extra money on Positioning 16
Aero Fork 14
Integrated Aerobars 9
Wheels (front and rear) 7
Aero Frame and Frok 7
I don't agree 100% with the list above, but I do think the first three are spot on.
For Clothes find a closeout trisuit from 2007 or 2008
For tires, here is some rolling resistance data: http://biketechreview.com/tires/imag...sting_rev8.pdf
When choosing tires remember that in general a wider tire is less aero, so a lot of people run a 20-22mm tire up front and a 23mm tire in rear. Also you'll need to weigh puncture resistance against those factors as well.
Aero Helmet - not all aero helmets are created equal. The Louis Garneau Rocket(without all the vents) and Giro Advantage are two good helmets in partcular.