Do SLI or CrossFire always improve performance ?
Not Always , there are some games that dont take advantage of SLI or CrossFire , so you wont see a difference between 2 cards and 1 card in those games , but in games that take advantage of SLI or CrossFire , 2 cards will perform better than one.
Is it really necessary to use SLI or CrossFire ?
Well , it really depends mostly on the resolutions and games you play , resolutions like 1920x1200 and higher benefit more from SLI or CrossFire than resolutions like 1600x1200 or lower (I dont mean SLI or CrossFire wont be good for resolutions like 1600x1200 or lower , i am just saying that SLI or CrossFire shines in higher resolutions.)
Also as i said , it depends on what games (with what settings) you play , for example if you just want to play games at good settings (with not everything at maximum settings) then MultiGPU technology isn't needed , but if you want to play with every option at maximum settings, then SLI would help , but again i repeat , resolution is very important.
Caution: Due to some problems (like Drivers and etc) in some games, a single card (like X1900XT) may beat 2 1900XT in CrossFire mode! so dont always think that 2 cards will beat one card.
Should i get 2 High-End cards or one Ultra-High-end ?
This is again one of the most asked questions. (The time i am writing this guide , The best NVIDIA cards are 8800GTX and 8800 ULTRA). Many ask that 2 8800GTS 320 will cost like a single 8800GTX 768, which is a better buy?
Well, in some cases , 2 8800GTS 320 cards will beat a 8800GTX cards(Mostly in games that dont need high memory and also in games that benefit MultiGPU technlogy). But in some other games (Mostly the games that need more memory to run better) a 8800GTX beats 2 8800GTS 320 in SLI mode.
Also remember that 2 8800GTS 320 cards will be more noisier and also they will produce more heat than one single 8800GTX.
if u play @ 1920x1200 or higher, definitely go for 8800GTX, if u play @ lower resolutions, go for a 8800GT or the new 8800GTS 512