I'm in the same boat (and will be rebuilding from scratch) - though I've just bought 256MB of PC133 SDRAM, I will probably go with whatever comes first
I am also in the rebuilding. Um, actually just building because I don'e have a unit yet. But through my constant search for info before I build my system, I came across some info about the A7M266 located <A HREF="http://www.asus.com.tw/whatsnew/whatsnew2010.html" target="_new">here</A>.
I know it is not much but I think it will help you in your decision. You said that you have just bought a 256MB PC133 SDRAM. If you intend to use this just go for the A7V133 because the A7M266 only supports PC1600/PC2100 DDR SDRAM (I got that info from the ASUS site).
I personally would wait for the A7M266 because I want higher bus speeds and I think it can be overclocked upto 360MHz(180x2). Also, I think this would be compatible with the upcoming AMD Palomino which I intend to buy (hopefully) as soon as it is out.