Cheapest would be GF 5200 but also slowest and limited to VGA only.
Get a R9700/9800 on eBay. Any reference one would do. See TME buying guides:
http://themacelite.wikidot.com/ati-radeon
Personally I'd recommend Hercules 9700 or 9800 Pro. They do have 128K EEPROMs so they can be flashed with full ROMs.