Hello. I've checked dozens of these 'spec check' sites to see if my pc is up to speed to run Oblivion and they always check out fine or better than needed. In fact, at times the games has run very well, then just...locks up and lags me too death.
My specs are as follows:

ASUS DeskTop PC CM5571
Pentium Dual-Core E5400 2.70GHz
6GB RAM
64-bit Operating System (Windows 7)
NVIDIA GeForce 8400 GS
HL-DT-ST DVDRAM GH40N ATA Device (DVD/CD-ROM)

Not sure if this is enough info or even the right info overall for anyone to make headway with.
I reinstalled to C:/Game Folder like I saw recommended on another site since it seems to help with Windows 7. It did, for a little bit.
My monitor is a 1600x900 widescreen, though I try to play the game at 1280x720, also widescreen, since I'd rather not have my characters dwarfed with the squish.

Anyways, as I said, my system checks out from what I've searched for, but it still has terrible issues.
Any help and/or advice would be greatly appreciated.
Also, thought it might help to state I've patched the game with Official 1.2, Unofficial 3.2 with Hotfix, and Unofficial SI 1.4

What kind of trouble? Slow frames or choppy? What?

Cant help if you don't give any useful information about your problem.

Also, your CPU and graphics card are weak.

I think it looks pretty good. Well enough RAM which was my big issue apart from the graphics card which I think you′d want to upgrade.
Also, are you running it with everything maxed out such as distance, AA, grass etc?

IMO, the 8400 GS isn't good at all compared to the "practical" minimum. Yes, it's better than the "official" minimum Geforces, but all of that was pure, absolute lying pie in the sky BS back then, which required a special "Ultra Low" setting to be added in the first patch. However, by choosing low screen resolutions, no higher than 1024 by 768, or the equivalent wide screen number of pixels, an 8400 doesn't have to bog down in displaying animation.

My bad. Yeah, it's choppy and the fps seems to go through the floor. I stagger with the mouse just trying to click any options on the creation menu. So upgrading my card would help? The people from Best Buy said that was a really good one to run it. Kinda thought they were full of it, but I stopped keeping up with what was best etc. a few years ago, so i don't really know these days. Thanks for replies so far.

Well, you supposedly have a PCIe-X16 slot, so, a vid card upgrade would be doable... 300 watt power supply might be a sticking point though... maybe a low-power consumption version of the 5770? or similar card? Would be a MAJOR upgrade for performance......