The specs are really only - a computer with asp.net 3.5sp1 or higher, and ms sql2005 or better.

Other than that, as ms sql works better the more memory is has access to
 - I would suggest you are better off on a 64bit system with 4gig of ram or more.

In terms of cpu / processing power -unless you expect to be handling millions of transactions a day
 - any on the current run of the mill processors will do the trick.

But to be very honest - these days I tend to advise people away from hosting their own websites.  
It tends to be all the support infrastructure that people tend to forget about when doing it themselves.
Backup / Fallover / UPS type systems that you need when you going it yourself.

For 20 or 30 dollars a month on top class hosting company you get all this and more.

For example - one of our client sites was being difficult recently - odd crashes and fallovers.
Within 2 hours of talking the issue thru with our live support contact the entire hosting was
migrated to a new server.

Doing that yourself - on your own system - well - start with the cost of purchasing a second server
and work up from there.

Oh and if you DO need a high through put server - have a look at the new cloud hosting systems
from www.powerdnn.com.

Cheers
westa