Do you have integrated video or sound? Remember they share the same memory (RAM) as the program runs in and its data is stored in, and only ONE device can access RAM at a time. With three devices (CPU=Central Processing Unit, GPU=Graphics Processing Unit and DSP=Digital Sound Processor) contending for the same RAM, 2 devices have to wait while 1 is busy, so there are serious contention bottlenecks which reduce framerates.
By all means buy an off-the-shelf PC, but be aware that manufacturers aim them at GENERAL PC users who usually don't play 3D games that stress the system too much. As such, manufacturers typically don't fit dedicated video or sound cards, relying instead on integrated video/sound on the Motherboard. This keeps the price of the PC down, making it more attractive to buyers and thus increasing sales.
Gamers typically upgrade to dedicated video/sound cards to relieve the bottlenecks. Each card is effectively a separate computer with its own on-board processor and memory. The CPU, GPU and DSP run in parallel with each other. The CPU is still in overall control but, once data is uploaded to the card memories, they can get on with their jobs at full speed while the CPU gets on with its job at full speed. Contention is minimised, which greatly improves framerates.
Integrated sound and/or video are common causes of low framerates and stuttering in Oblivion, and it's due to bottlenecking. The Quiet Feet mod can help by reducing sound data traffic, and reducing graphical detail (especially Texture Size) and using optimised meshes can help by reducing texture/shader/geometry data traffic. However, if you have a Desktop PC, there's nothing to stop you fitting dedicated video and sound cards to relieve the bottlenecking.
If your video card has one or more additional power sockets, do you have it/them plugged into the PC's power harness? If not, the AGP or PCI-E slot can only supply a limited amount of power, insufficient to run a modern video card at FULL speed. As such, the card will automatically throttle its clock speeds down, GREATLY UNDERCLOCKING itself to reduce power consumption, and this has MAJOR effects on framerates.
Does your Power Supply Unit (PSU) have sufficient wattage to supply the system with enough power? If not, your PC will be unstable, likely to reboot itself (brown out under load) when running games that stress the system, or subject to frequent Blue Screen errors (fatal crashes). Does your PSU have multiple +12V rails? If it does, make sure the video card's +12V additional power is from a separate +12V rail to everything else, and other devices are power-balanced. E.g. Motherboard on one rail, drives on another. Modern PC's usually have PSU's with at least THREE +12V rails for this very reason (1 for mobo, 1 for drives, 1 for video).
For high-end video cards, I suggest at least an 850W PSU with 3 +12V rails. If you're running 2 video cards in SLI mode, I suggest at least a 1000W PSU with 4 +12V rails (mobo, drives, video 1, video 2). Higher wattage is better. The PC will only draw as much power as needed, and there's plenty of "headroom" for future upgrades. Plus running a higher wattage rated PSU at lower wattage means its under less stress which extends its expected operational lifespan. Typical PSU lifespan is around 80,000 hours of continuous use, but cheap PSU's "age" more rapidly over time, reducing their lifespan. Bad PSU's might be rated for 80,000 hours, but that's their predicted lifespan under NORMAL stress, i.e. 2D Desktop work. They might only last 30,000 hours or less under high stress conditions, like running 3D games. Quality is the key. You pay for what you get. High quality PSU's cost more but "age" more slowly and have longer more stable lifespans.
Note: PC manufacturers often skimp on the PSU, fitting cheap ones to save build costs (lower build cost = higher profit). The PSU is often the OTHER most common part that dedicated gamers end up upgrading. If you're not PC-savvy, the PSU is often overlooked when fitting a modern high-end video card, and instability or crashes are the result. The cure is to upgrade to a more powerful PSU and make sure the +12V rail loads are kept below their maximum ratings. The above suggestions are common easy ways around having to use a meter to measure loads. There usually aren't enough drives fitted to overrun one +12V rail, and ensuring the video card is on a separate +12V rail means nothing else can suddenly increase the load on that rail and overrun it, such as an optical drive spinning up. That way, the system remains stable even under maximum stress.
Hope this helps.
