After way too much thought, I think I am going to end up staying with the 406 size wheel. There just doesn't seem like a big enough improvement compared to cost to jump to the 451s. However, I do have a wider rim and was wondering if anyone has suggestions for either prebuilt wheels or narrower rims that would allow me to use Swalbe Stelvios or the skinny Contis. I am interested in lighter rotating mass and skinnier tires..I still want to make my Raleigh Twenty faster:)!!!
Juan

I feel as though I am heading in the same direction as you...Just getting info on 451 related wheels/tires seems to take twice as much effort as 406.

Gaerlan (http://www.gaerlan.com/bikeparts/parts/406wheel/wheelbmx.html)has some 20s (406 and 451) in stock. Your LBS should be able to procure the rims and hubs you need then build the wheels for you. But they are probably not going to have anything in stock.

Best source of 451 rims/tires/tubes is bike friday! They have it all listed under 'accessories'. Call them and they will sell you at decent prices. BTW, 451's make a difference in ride quality, speed and aesthetics.

Here's a 20 on eBay with 451 rims. He also invites questions:
Raleigh 20 (http://cgi.ebay.com/Raleigh-Twenty-folding-bicycle-project-in-progress_W0QQitemZ220004856263QQihZ012QQcategoryZ98084QQrdZ1QQcmdZViewItem)