My last two bikes have been previously loved Roubaix frames from ebay and it's worked out great both times. There's a lot of FUD surrounding used carbon frames, some of it justified. Obviously you lose out on any warranty coverage, so the discount from new should be significant.

Another route if you don't feel totally comfortable going the China route is to get a nashbar or pedalforce frame. $600 isn't terribly more than the direct china price and you get to deal with an actual "local" company rather than some potentially iffy chinese trading company.

I personally haven't purchased a complete bike since 1992 - I prefer to selectively upgrade stuff. I realize that isn't necessarily the most cost efficient way but it spreads it out nicely.