A good card is an x800 GTO if you plan to go with the x800 family. It can be had for about $150 US and is much better than your 5900 XT in Oblivion. I think the x850 is only PCI-E though. The x800 series can be found in AGP. If you can afford it, the 7800 GT in AGP would be the best AGP board you can get.
CHeck the review:
http://www.tomshardware.com/2005/11/28/don/
And here are some benchmarks from Anandtech:
http://www.anandtech.com/video/showdoc.aspx?i=2746&p=7