The ATI Radeon HD 3870 you purchased may be a PCI-Express card and not an AGP graphics card. Your computer may also not have a powerful enough power supply since the 3870 requires, at an absolute bare minimum, a 450 watt power supply. I don't believe there is an AGP to PCI-Express card, there is a PCI-Express to PCI card on the market, but that won't help. There is a VGA to S-Video adapter (similar to the DVI to VGA adapter that comes with graphics cards) you can get, but they are often hit-or-miss. You can get a converter like the KWORLD PlusTV PCTOTV Converter SA235 USB 2.0 Interface
. It, obviously, won't do HD, but it will take VGA and output S-Video or composite.
Edited by DJBPace07, 05 October 2009 - 11:39 PM.