Recommended Specs:
* 3 Ghz Intel Pentium 4 or equivalent processor
* 1 GB System RAM
* ATI X800 series, Nvidia GeForce 6800 series, or higher video card
Minimum Specs:
* Windows XP
* 512MB System RAM
* 2 Ghz Intel Pentium 4 or equivalent processor
* 128MB Direct3D compatible video card and DirectX 9.0 compatible driver
* 8x DVD-ROM drive
* 4.6 GB free hard disk space
The following video cards or video card series will also be supported in Oblivion:
* ATI X1800 series
* ATI X1300 series
* ATI X850 series
* ATI x800 series
* ATI x700 series
* ATI x600 series
* ATI Radeon 9800 series
* ATI Radeon 9700 series
* ATI Radeon 9600 series
* ATI Radeon 9500 series
* ATI Radeon 9000 series
* NVIDIA Geforce 7800 series
* NVIDIA GeForce 6800 series
* NVIDIA GeForce FX series
I personally would take Windows XP being the required operating system with a pinch of salt. Historically speaking, anything that "requires" XP will also run on 2000, and there are often unofficial workarounds to get it working on '98 (as was the case with Doom 3).