With the exception of Doom 3, I don't think there is ANYTHING going to come out that won't run like a mad train (ie, very good) on a Radeon 9700 or a Geforce 4 Ti4600.
Even Doom 3 will run fine on those (providing you have enough processor power), but you won't get to see all of Carmack's Shiny New Toys. Then again, unless you have a cluster of SG workstations, I doubt ANYTHING is going to run Doom 3 with all the bells and whistles on at more than 5fps.
Let's not forget that Carmack said that he expected the Doom 3 engine to last for five years.
So, if you want to upgrade to something spanky, I'd get a Radeon 9700. 1600x1200x32 at 100+fps is sweeeeeeeet