It's going to be hard to find a $150 AGP that can run Oblivion at high settings, especially at 10x7 resolution. Med-high no FAAA is more probable. Just make sure the rest of the system is up to spec also.
The best AGP you are going to find for that price is a X800GTO 256MB. It performs as well as a 6800GS, which are more expensive ($200-220 now) and slower clocked than the PCI-e versions. And $180 won't make a difference as unfortuanately their just aren't many options for AGP. To do better you need to spend $270+ on a 7800GS.
Basically in AGP here is a rough pecking order, just in case you can find one on the list that's better than a X800GTO and priced in you budget. Example, a X850 pro would be a step above the X800GTO and some COmpUSA retail stores had them for $179 a short while back.
GF6800/X800 GTO 128MB (slower mem clocks than the 256MB version)
GF6800GS in AGP is going to run you $200 plus now. It has SM3 but is not faster than a much cheaper X800GTO 256MB. The X1600 pro is cheap, $120-$130, but would get blown out by a GTO.
We don't have Oblivion benchmarks for these cards. But here you can find other benchies. You'll see that card #19 a 12-pipe 6800 256-bit clocked 350/1000 is identical to a 6800GS. In most benchies they did, the X800GTO performs higher.
I have an AGP X800GTO. It is a pretty good card and has potential for overclocking and/or unlocking. The only disadvantage of the X800 series is that they don't support Shader Model 3. That doesn't matter too much now but in the next year or so it might start making a significant difference.
The card I have the memory overclocks a fair amount but the core doesn't. I did, however, modify the bios so that it now has 16 active pixel pipelines.