For what L1qu1d described, I would suggest just leaving the 4870 as is and just running standard ATI Catalyst drivers. Modding it into a FireGL, if successful, is fairly risky and you may end up with a cool paper-weight.
If you want an actual FireGL, you can find them on eBay and Craig's List fairly priced.
I would also be willing to trade a FireGL V3600 I have for your 4870.
Bottom line, the 4870 will do what you described for the GF reasonably well.
HOOAH!!!