Yes, you can buy an SLI/CrossfireX graphic card to be used in your regular (non-sli/cfx) computer. You can even buy two of them to support multiple monitors. They just will not work together in SLI/CrossFire mode.
What about power supply? When looking at cards requirements, it say I need a SLI compatible/supported power supply.
That's only if you are planning on using multiple GPUs, which will usually indicate the use of Crossfire/SLI. If you're only going to buy 1 GPU, then you only need a PSU which will support the once card (Although it's a good idea to buy a better PSU than minimum in case you want to upgrade some time.)
cfx/sli ready doesn't matter if your motherboard or your pc doesn't support cfx/sli. sometimes gfx card w/o cfx/sli support cost a little less. single cards also eliminate the potential hassle that involves setting up multi card setups.
single cfx/sli ready gfx card + mobo w/o cfx/sli support - will work.
single cfx/sli ready gfx card + mobo with cfx/sli support - will work. advantage - ability to add another card down the road. exception: if the second pcie slot has x4 bandwidth, then it is not advisable due to bottleneck issue.
cfx/sli faq: http://www.tomshardware.com/forum/245454-33-crossfire-f...
but it's good to have support for them. if you already possess a good psu you could use that on your next build or on another pc and save some money. the psu's ability to be able to supply continuous power with proper efficiency on load takes priority.
psu info thread: http://www.tomshardware.com/forum/284314-28-power-suppl...