I have a quad core, 2.80ghz with 4 gig RAM, Windows 7 64bit I bought from Dell last spring and paid $500 for the cpu. Obviously you will pay more with a new monitor. I have a 19" HD monitor I paid $100 for from dell as well.
I have no trouble editing 1920x1080 files with vegas. I was using MSP before I upgraded to Pro a few months ago and I did have to do a RAM modification to work with more than a few files. I found the directions for the modification on this forum site. I did not do anything when I went to Pro and it works fine. Make sure you have at least 1 external drive to keep your raw files on so you are not using up the cpu's storage space...good to have a few for backing up files anyhow.

"I have no trouble editing 1920x1080 files..."

Not all 1920x1080 files are AVCHD. Are yours? Mine aren't.

I have 2 HD cameras. One Sony that shoots AVCHD and a Samsung that shoots H.264

I do not have any trouble editing or rendering either format with Vegas 10 and the system I detailed above.

I have an Intel Core 2 6700 overclocked to 3.2 GHz with 6 GB of memory. 64 bit WIndows 7 OS. Vegas 10 Pro.

Is that sufficient to edit AVCHD?

I think you should just try and get a few clips and try it out - it's sometimes hard to judge performance based on others experience.

Ulf

that will be adequate, but you'd be better off with a Quad Core for reasonable rendering times. That's an easy upgrade and still overclockable. Q6600 or Q6700 should run 3.2 ghz or better, mine is running 3.33ghz and is very stable.