The only reason id see to upgrade to a newer driver is due to something like geometry instancing in farcry. That is, if you are like me and using the 1.2 patch (even though it was recalled but appears fine to me on a fresh install even after going through 1/2 the game) you will notice an increase in your framerate in farcry and potentially any future game that will be using geometry instancing. The 4.8's are the first drivers that enable the 9800p to use geometry instancing and i believe they also provide some minor doom 3 increases in framerate.
Sorry for that rant above, but basically the summary is that i reccomend the 4.8's due to them having the most bug fixes in current games, as well as you being able to use geometry instancing in current and future games. As far as older drivers go, they all have minor increases/decreases in framerate depending upon the game you play. The newer the driver, the more stable it will be (usually).