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.)
So i dont recommend CrossFire for your resolution, also :
A note about Motherboards:
Some Motherboards support Multi-GPU Technology at Dual 16x mode, some support it at Dual 8x mode and some support it at 16x 4x mode, what are the differences?
Well for gettig the best performance out of Multi-GPU,you need a Motherboard that supports Multi-GPU at
dual 16x mode like Nvidia 780i SLI, NVIDIA 680i( As i said, NVIDIA 780I and 680i support 3-Way SLI too),Nvidia 590i SLI, Nvidia nForce4 SLI 16x, ATI CrossFire Xpress 3200 or Intel X38 chipsets.
Dual 8x is a very good configuration too,it doesnt have alot of difference with Dual 16x and its cheaper too.
Chipests like Nvidia 750i SLI, Nvidia 650i SLI,NVIDIA 570i SLI, NVIDIA nForce4 SLI, ATI Radeon Xpress 200, Intel 975x and ONLY 2 Intel P35 boards(ASUS BLITZ FORMULA and ASUS BLITZ EXTREME) support Multi-GPU at dual 8x mode.
The last one is 16x 4x, this one is the weakest and doesn't perform very good compared to dual 8x or dual 16x,so i won't recommend it for Multi-GPU configuration. Intel P35 and P965 chipsets support Multi-GPU at
16x 4x mode.
Dual 8x vs 16x 4x (ASUS P5B DELUXE vs P5W-DH DELUXE)http://www.anandtech.com/video/showdoc.aspx?i=2837