what presentation application needs MS-SQL? or is this some custom thingy?
having used MS-SQL extensively for several years in my line of work I can tell you that RAM is your biggest need, not raw processing power. that database is only gonna be serving data to a single use (itself) so CPU grunt aint vital.
having said that, the EPIA 800/900's should both be fine IMHO, just give it 256/512MB of RAM for the task.
personally im getting away from the MS side of things for web applications development nowadays, launching a big corporate site on the 1st Feb which I have developed ground up in PHP/mySQL all the way.

MS-SQL 2000 was wayyy too much trubble for me... even tho you miss out on transaction support (until mySQL 4.0)