I have some direct experience that hopefully will help you in your decision-making process. I was running Oblivion with a AMD 3700 (Clawhammer)/9700pro setup, before upgrading to the x850xt.
If you look at the Oblivion CPU article here on AT, my 3700 would fall somewhere between the s939 3700 (at 2.2Ghz/1MB L2) and the 4000 (2.4Ghz/1MB L2)--or, quite honestly, very close to either the 3800 (2.4Ghz/512K L2) single or X2 3800 (2.0Ghz/512K L2 x2). If you cross-reference these charts versus the new benchmarks that compare Conroe to the AMD offerings, you can get a good idea of the new price/performance ratios come 7/23. While availability and pricing may prevent using a Conroe (cheapest is around $180 list, IIRC), they still will push the prices of all other CPUs, AMD and Intel, down dramatically. You may find that the price cuts will impact what you feel will be your best price/performer.
In general, I think some of the folks advice about prioritizing GPU performance is merited for most games. Oblivion, however, is a peculiar bird, as these AT charts indicate:
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2747&p=4
Summed up, they indicate that outdoor settings in Oblivion (the Great Forest, Oblivion Gates, etc.) are heavily GPU dependent, while "indoor" settings (town & dungeon) are more affected by CPU performance.
Yet, the maxim of prioritizing GPU still applies here for the most part. If you look at the average frame-rates achievable by lower-clocked CPUs in town and dungeon settings, they're still acceptable (fps in the low 30s), while the wrong GPU begins to make certain settins in Oblivion unplayable (dropping well below 20fps).
I have a 3700 Clawhammer paired with an x850xt and generally Oblivion performs well at 1280x1024--with a lot of .ini and graphical tweaking. The fact is, right from the box, very little below the x1800xt/x1900xt(x), 7900gt(x)/7950gx2 can really plow through Oblivion at resolutions higher than 1280x1024 with all of the settings maximized at default .ini settings. You can check out the AT article on Oblivion GPU performance here:
http://www.anandtech.com/video/showdoc.aspx?i=2746
Those charts went a long way towards convincing me to choose the x850xt. Although it can't run SM3 and true HDR in Oblivion, most of the cards that can (7600, x1600, x1800gto) in the mid-range price (~$100-200) perform a decent clip below the x850xt when you do enable HDR and don't necessarily beat it when it comes to Bloom. As you can see, at default, the x850xt will generally outperform the 7600 with Bloom enabled:
http://www.anandtech.com/video/showdoc.aspx?i=2746&p=5
While the x1800gto does beat it out, it also costs more. Neither one, once you enable HDR, really achieves acceptable frame-rates, coming in at around 15fps give or take.
In all of this I'm basically ignoring the Town and Dungeon benchmarks and focusing strictly on the Oblivion Gate benchmark. Why? Because all three cards (7600gt, x850xt, x1800gto) do a very fine job giving you acceptable frame-rates in those settings. The 7600gt comes out ahead in some, even. But the difference between averaging 20fps, with a minimum around 15, in the high-stress situations (for the x850xt) versus averaging 17, with minimums in the single digits (for the 7600gt) was more important--to me--than averaging 73fps (7600gt) over 67fps (x850xt) in town settings.
In the end, I'd balance your CPU/GPU choice with two or three things in mind:
1) Prices are dropping on 7/23 and all the price/performance ratios will be affected.
2) After 7/23 there may be no reason not to consider dual-core. The slight benefits of a faster single-core CPU for some gaming (IF you pair it will a GPU that can bring out those benefits) may not outweighed the improved responsiveness of a dual-core machine while multi-tasking with your normal applications.
3) Decide how much running Oblivion well means to you right now. I'd say that the 7600gt is a better GPU choice, in some sense, than an x850xt. I don't think it's a better choice for Oblivion, however.
Personally, I didn't want to spend hundreds of dollars on a GPU--nor did I want to wait a year for a $150, Oblivion-SM3/HDR-capable GPU, in order to play Oblivion. I got the x850xt and have been--in the main--happy with it. It's not going to run the game like an x1800xt/1900xt, but it doesn't do badly. It was a significant step up from my 9700pro.
Hope this helps! If you actually want specifics about my Oblivion experiences (resolution, settings, fps)--just let me know.
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