For modded oblivion (let's assume with quarl's texture pack 3) you should aim for a near 3ghz processor
CPU
architecture trumps CPU
clock speed, to some extent. I wouldn't bother looking at clock speed, unless you're into overclocking, perhaps.* These days, the speeds are all pretty good, even at the low end (e.g., Athlon II or i3). Architectures are also all quite nice, though some outweigh others. Personally, I'd be tempted with either an i5 from Intel, or Phenom II from AMD.
* But even then, multiplier is the thing...
Anyway, you're much better off building your own. It's not very difficult, and people on forums and elsewhere should be willing and able to help if necessary. But if you want to pay someone to build for you, make sure you pick the best possible parts at good prices, then sure, hand over the extra $50 or whatever it is.
Graphics card requirements depend on, among other things, the resolution at which you will play. Of course, the mods you'll insist on using, and/or things like AA and AF, have a huge impact as well.
I make do
with an HD5870 (overclocked by me, but only a hundred MHz or so), and an overclocked e8400 (an old CPU, yes), running at 4GHz. I run a lot of graphics mods, to put it mildly, including effects such as those from OBGEv2. Never have issues with FPS, suffice it to say.
So, perhaps that will give you an idea or two. If I was buying (i.e., building) new right now, it would be an i5 760 (overclocked),
at least a 1GB GTX460 (OCd by me - these are
amazing when boosted a bit) - or maybe something pricier, a fast SSD for OS and apps, a nice HDD for storage, a Corsair PSU (like I have now, but with more power), a Black Pearl case, and a Dell high colour gamut monitor. But that might go over your limit, so - given your budget - I'd adjust to taste.