The 9800GX2 added 4-6fps at the low end, which equates to almost a 10% increase. In the mid-range the performance increase grows, adding 10-15fps. The equates to a 25%-33% increase. The 4M pixel range adds 20-25fps, which is right at a 100% increase. The 9800GX2 certainly improves performance at the higher end, keeping the average frames at a playable 50fps.
But, after tooling around Oblivion for a bit, I have to say I am dissapointed - not in the 9800GX2, but in Oblivion. The game seems to have some sort of 60fps cap on it (after Googling, I found a number of people who think the same thing), and the game "chunks" at the same point every time. Even with the 9800GX2 at the lowest resolutions, I could still bank on the spots where the frame rate would "hitch" and go south of 30fps. That, and I hate wolves. I don't know how many times I had to re-start a run, due to some damn wolf that chased me for miles.
Oblivion is a punishing game, and there are fan-made mods that make it even more so. I installed all of the "highly recommended mods" from Tweak Guide's extensive article on Oblivion (even the 2GB of high quality textures). I then downloaded an end-game save, so I could teleport around to find a good run, where I could put the cards through their paces.
I found a nice long run (about six minutes), that crossed a large lake outside the capitol, then circled through the woods, and ended at a look-out over the same lake (looking at the capitol). This run goes through a tone of foliage and scenery, and I was able to hit some cloud cover that changed the entire light map from normal sunlight to a red hue. That was pretty cool to watch. I still hate wolves, and I don't know why they chase a fully armored knight for what seems like miles, and I don't know why a few swings of a sword doesn't put them down.