FWIW, I've found that Oblivion can be very variable.
I haven't played Oblivion in a couple months, but when I was playing, I was playing with these specs:
Athlon 64 3500+ (o/c from 2.2 to 2.3)
2gb ram
X800XL 512mb
I played at 1024x768 with the sliders in different places, but overall it was roughly a "medium" quality level. I also had a few of the enhanced texture pack mods installed. I had tested with and without these and didn't really find a noticable framerate difference (it seemed to be like 0.5-1 or so). Can't remember the names offhand, but there were a couple parallax packs and LOD packs.
Most areas I would get doable framerates (25+) indoors and outdoors. However, there is a mountainous area to the north where my framerate would dip pretty poorly, about 15.
There is also the area to the west near Anvil with all the tall grasses. In that area I would get pathetic 10-15 fps. Even turning the grass distance down didn't help much.
Also, sometimes in dungeons when more than one monster was nearby (even if not actively attacking me), my sound would start stuttering like crazy and my framerate would drop. I minimized this by some ini tweaks to turn off hw acceleration for sound and lowering the Max Impact sound count.
Many ini tweaks can be found here, starting on pg 8:
http://www.tweakguides.com/Oblivion_8.html
Overall, Oblivion was playable with the above specs, it's just in certain areas where it would drop and become a chugfest.
Anyway, I haven't played Oblivion in a while. My x800XL crapped out, so I replaced it with a 512mb x1650Pro as surprisingly, x800XLs were going for almost as much as when I had bought mine like 2 years before, and I needed a cheaper replacement.
I will very soon be building a new box (parts on order) so I'll look forward to any improvements.
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