Any low pro AGP card will work. Best bet is probablly the 3450, but they cost a small fortune now.
I got 4 HPD530s 3 years ago. They are great machines. Mine all had the P4 3.0HT. with 4x512 for 2gigs of ram. Back then I was able to get an agp ASUS 3450 with 512 megs of ram and HDMI fo $40.
I took the floppy drive out, and the green internal handle to free up some space. Did a clean install of win7 32 (beacuse the p4 is not 64 bit) Then installed the low pro 3450. They make great HTPCs.
Last year I got my hands on a pair of HP DC7700 mother boards and Chips. The snap in metal motherboard tray fits into the D530 perfectly. So now my DC530s have core 2 2.0 and PCI-e cards.