My specs are a little different than yours, but fairly close.
It's an "old" M170. Has the single core 2.1, 2gb ram, and a 7800 go gtx.
I run the game at native resolution (1920x1200). Almost everything maxed.
I have all the sliders set to the far right side except shadows wich are at default.
I have disabled shadow filtering. I'm still swapping back and forth between HDR and Bloom. (seems a little, to overexposed, still playing with the ini to tweak to taste)
I have several texture mods that up the resolution of textures in game. The causes me to have just shy of 500mb of video textures loaded at any given moment. (card has 256)
Oh, I also run with 6x AF and have lowered the amount of grass but increased the distance.
Outside I stay between 20 and 30fps. In cities its closer to 20 most the time (outside averages about 26). In dungions I get an extra 10 frames or so, mostly between 30 and 40.
Needless to say the game looks Stunning. When I travel on horse I may stutter every now and then (because of the massive textures and AF). I'm crazy about great looking sharp textures so I'm willing to live with it. If I set the textures to medium (all mip mapped images half both vertical and horizontal resolution) the game still looks great and stuttering is almost gone (video memory is down to about 220).
In combat there is not stuttering and frames bairly drop (no loading).
I play a heavily modified version of obivion so, there is a lot more in my game than normal and my comp handles it great.
As to not playing at native resolution, I can tell the difference. Again, I'm crazy about sharp textures (I hit the AF before the AA). It is bairly noticeable though so you will still enjoy the game at your chosen res. A note about the UI, it scales to your resolution, so there is not the problem like in other games where at a high res you can bairly see the UI (I run normal windows programs like email and such at 1280x800 beause I like nice big font).
I also use the Dark UI (leather based ui with more info on screen).
Hope this helps gauge what type of settings you may be able to achieve. I think our graphics cards are about the same? My GHZ is higher than yours, but you got the duo (so enable multi threading settings in ui). Memory is the same, oh, I got the 7200 60gb hard drive so my load times may be a bit less.