Yes, but then you're upgrading a $1500 machine rather than a $2500 machine - and a Conroe box wouldn't require FB-DIMMs.
$1500 + 400 (X1900XT) + $100 (1GB of memory) = $2000. Right in between a well-equipped Mini and a Mac Pro.
My reasoning is that it's well and good to say 'you can get a Mac Pro for <X>' - but that Mac Pro is, essentially crippled and will either not suit your needs as long, or will require more money for updates in the future. The Mac Pro, well-equipped now or piecemeal, is a $3000+ machine - when you factor in extra hard drives, more memory (Leopard will be a hog, rest assured) a second optical drive of some kind, it's more like a $4000 machine.
Yes, Dell used to sell AGP machines - just like Apple (actually, Apple was selling AGP more recently). Then technology moved on, and their core 2 machines (more likely the entire consumer desktop line) are running PCI-E.