DVI-D cable connection to a Dell Dimension 2400?

I just bought a ViewSonic HD monitor VX2250 and want to connect it to my Dell Dimension 2400. Monitor comes with a DVI cable but cant connect. New video card or is adaptor possible? I know the computer is ancient but my young kids use it and wanted to upgrade the monitor for them. Not a tech person and need to know easiest and least expensive way to do this. Thanks.
 
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