I stick to taking taxis -- or (better yet) calling up radio taxis -- around Delhi when I don't have a car. The same goes for pretty much any large city in India.

There is no rail line from the airport to Agra. You would have to use a regular railway station rather than the Delhi metro to get to/from Agra.

When DEL hotels are overpriced nowadays, there is the option of a pay-by-the-hour hotel facility at the airport itself. Combine that with a hired car for the hours there, and there's more bang for the buck than paying an inflated price for the Radisson in DEL.

[The Radisson Delhi (closest to the airport) is one of the properties where I have had the worst luck in trying to get in on a certain promotional rate discussed in the Hotel Deals thread relevant to the family of hotels. It works for the Radisson in Connaught Place quite regularly, however. Hope that helps even as it won't do anything in terms of frequent guest program credit.]