Is there not a dedicated Oblivion Forum?
The reason I suggest this is, this particular game, which I have not tried myself seems to need the most powerful hardware imagineable. The amount of people who have really decent PCs and struggle with this game are common.
Personally I think upgrading what you have might be a waste, it depends on if any upgrade could be used again, since you say you have everything on max I would tweak it a bit, drop some of the setting a tad until you are happy with the framerate.
My computers too low spec for this game thats the main reason I've never tried it.