thanks for all the input. sounds like maybe 48 or 50 mm is the way to go.

WCSting - on a road bike, with no aero gear whatsoever, i usually avg 21-22 mph for TTs and tris (as long as the course isn't a complete roller coaster). if i try to get really aero, though, i can tell the road bike setup isn't optimal and i can't put out power as efficiently. i'll probably get a tri bike soon. longest tri i've done so far is olympic distance, but i will be doing longer in the future. i'm not necessarily looking for the absolute cheapest option but zipps are really out of my budget.

AdelaaR - thinking about doing that, except with a disc cover instead of an actual disc.