Then again, maybe you can't spend any cash at all to upgrade your rig right now. If you know you're stuck with the PC you have and want to make Oblivion run better on whatever you've got, here's some advice, though you may not like it. First, the lower your resolution, the quicker the game will run - that means lowly settings like 640x480 and 800x600. Be sure to turn off “Shadows on Grass,” and “Shadow Filtering,” two big-time performance killers for older rigs. And anything that has to do with displaying objects in the distance should be cranked as low as you can go, too. After all, who cares about the forest on the horizon if you can't see the trees in front of you?
If you’d like to do more heavy-duty tweaking, first look at the readme.txt file that's sitting in your Oblivion game directory; it has a lot of simple checklist kind of things that can help. To get more intense, check out the official Oblivion website at http://www.elderscrolls.com/forums. The FAQ is a must-read, offering tons of tweaks (some get pretty technical, so proceed with caution), as well as some easy fixes to bugs that hadn’t been ironed out at launch. Happy questing!