Where to begin? lol.
From experience, multimonitor setups has pros and cons.
larger real estate,
games look fantastic.
bezels can be distracting (personally I don't notice this),
games may not scale properly (ie poor performance)-the VGA is trying to process 3 times the amount of pixels,
games (ex FPS) may look "squashed", needing an FOV hack.
For multimonitor setups, it's best to have the same brand/res to reduce conflicts. Although I have listed more cons than pros, get it right, and you will never go back to single monitor setups.
ATI or NVIDIA? I have tried both, Eyefinity and NV Surround. I gave up using Eyefinity because of driver issues and poor performance. I was an early adopter, but IMO, active displayport flickering, powerplay issues, poor performance made me trash the lot.
NV Surround worked first go. Performance is great. Near perfect scaling.
As for other items:
1. I prefer Intel CPUs. Better performance. Wait for Sandybridge (i72600K).
2. Get an SSD. Two if possible. Capacity may be an issue, but worth the performance. Sandforce controllers have good performance.
3. VGA. ATI 6xxx series in crossfire or NVIDIA 5xx series in SLI. Quadfire/quad SLI scales terribly. Eyefinity you can run using a single card but you will run out of "juice" ie VRAM. NV Surround you need to run in SLI. You probably will want to run dual cards anyways to get the performance.
4. Motherboard-P67. Asus or Gigabyte IMO.
5. Watercooling-Good performance but IMO only use if overclocking/benchmarking. It can be unnerving to instal/maintain.
6. Get a good PSU. I made this mistake. A good 80+.
7. Get a good full size tower. Coolermaster (HAF X), Corsair (800D). Good airflow is a must. Cable management is important also.
If going for Sandybridge you will need to wait as it's just being released. Although reviews seem quite positive. Also, 6990 and 595 dual GPUs will be released Q1.
Personally, I am waiting for more reviews on Sandybridge CPUs, P67 motherboards and dual GPU solutions from both camps before upgrading. Q1 is going to be exciting.
Sites such as hardocp have good reviews on Eyefinity/NV Surround performance.
Keep to a budget. The best and most expensive today will only become "obselete" next month.
PS. Opinions are based on my own experiences. I am not intending to start flame wars or fanboyism.