With today's Mac Pro updates Apple began shipping a new graphics card as a build-to-order option. The $200 ATI Radeon HD 4870 option can be put in place of the standard NVIDIA GeFore GT 120 graphics card.
In addition Apple looks set to make the graphics card available as an upgrade to Early-2008 Mac Pro models. Currently the upgrade has no price point but will most likely come in around $350.
The upgrade is also listed as "Currently Unavailable" but the timeline of release is expected to coincide with the release of Mac OS X 10.5.7 which began seeding to developers last week.