Giving any kind of hardware recommendations for simulators is not easy, but there are few general ideas that are important;
- Spend your money on the CPU rather then the GPU, most sims are limited in framerate by the CPU and the GPU can make it look better, not a lot faster.
- SLI graphics hardware makes FS2004/FSX/P3D slower.
- 8 Gb of memory works fine, more is nice but it will not help your FPS.
- An SSD is the best investment you can ever do.
There are many guides on the internet that can assist you in selecting hardware. We like the guides at http://www.tomshardware.com/ a lot. search for 'System Builder Marathon". For Dutch users we also advise tweakers.net.
For your operating system things are far less complex, we strongly advise Windows 7 in the 64 bits version.