aerobars and other upgrades

recently got a nice bonus from my job and am looking to make a nice upgrade on my bike. aside from fit, wheels (too much), what would be a really good upgrade to help with efficiency and speed? also...if you could list the top 5 parts to upgrade (again aside from wheels and fit) what would they be...from best to worst.

also...been looking at aerobars...what is the benefit of going with the ones that stick straight out rather than the ones that curve up and loop around? i want to invest in aerobars...but i am concerened that if i go with one it wont be as comfortable as another. do bike stores have sets of aerobars ready to put on and try there to see what you like?

thanks