The turnout in 2004 was 460k and in 2005 it was 270k. My guess is that the turnout in this year's race will be somewhere between those, so lets say 365k voters in Fairfax this year. That would be turnout of 58.6%, which is reasonable. Based on this turnout number, the final count would be 232k Webb, 132k Allen and 1000 write-ins and indys. That would mean Webb would be getting 63.6%. For comparison, Kaine got 60.15% in Fairfax last year.
As Ben's post points out, this is the best case scenario for them, and it assumes Bush's poll numbers fall another 5 points or so. While I am not sure how those numbers can fall any further without him being revealed as satan (therefore losing the religious right's backing), I do think that a 100k loss in Fairfax is possible. This race will be tell tale sign as to whether Virginia is really in play in 2008, or whether we just like dem governors and GOP presidents.
I will have a few more posts related to this topic, such as the strategy of how Allen can win if he loses Fairfax by 100k, and also what the possible fallout will be in the event Allen loses this year.