Hi there
The dual Xeons will be fine. Asterisk won't use much more than half a Gig of the memory but it's always nice to have.
You'll probabaly have to mess around a bit with the Vegastreams to get them to register properly but we've used Multitech 8-port FXS equivalents on many occasions and they work just fine (in the end, SIP is just SIP).
We don't support the the Digium BRI cards, they will only run under mISDN and we don't currently have an implementation for it. However, we do support the Junghanns equivalents which are, in our opinion, better made and more functional. You might also consider using a Vega/Multitech/Patton BRI gateway if you don't like the idea of the Junghanns cards as a solution.
370 is fine.
No script for the extensions I'm afraid but, because it is all file driven, we could show you how to do cut and paste on the database which would still be pretty fast.
Will it work? Yes it will but you'll probably need to do a little bit of fiddling with the Vegas (probably a coupe of hours work).
Kind Regards