I picked up the Geforce 7600GT AGP for about $30 on ebay. You could also look for an ATI 9600Pro or above in AGP that might let you get by. I'm assuming you need AGP, so that limits your possibilities.
If NPVR offers the FSE renderer you might try that. It might let you get by.
HTPC: E7500, Nvidia 9500GT, ATI Theater 650Pro PCI/Firewire Network Recorder/Avermedia Duet A188, NPVR 2.6.2, Win7 Pro
Test: Asrock 890GM Pro 3, Athlon X2 255 , internal HD4290, ATI Theater 650Pro PCIe, NPVR 2.6.2, Win7 Home Premium
Lounge: Athlon 3200, Nvidia 7600GT, Network Recorder, NPVR 2.5.5, Win7 Home Premium