Crossfire & SLI, and other question

I'm looking into building a new computer and I understand that motherboards are either SLI or Crossfire compatiable. Can I just use a singleNvidia card in a otherboard that is Crossfire compatiable, for example..?

Also, what should I be looking at when choosing a graphics card, processor speed, memory speed, memory quantity? I will be using the computer for CAD and Photoshop. I know that I really want a workstation card but thats a bit out of budget! Upto £60 / $90 is my limit really. Thanks! Doug.

Difference between gaming and workstation cards
http://www.tomshardware.com/reviews/opengl-workstation-graphics,1269.html

There are no brand-specific limitations when installing single cards onto motherboards. However, when I installed an 8600GT on my family's computer which had an integrated ATi graphics chip, I had to disable the onboard video and uninstall CCC to get the card to work. Nothing too complicated though.

Processor speed and memory speed are almost self explanatory, but can't really be a factor when comparing different cards. My 8800GT is at 740MHz, a GTX 280 is stock clocked at 602MHz, but the GTX 280 will always beat my card by a long shot. Memory can be compared, but isn't a huge factor. The faster the better basically. Memory quantity helps stabilize and possibly increase your framerates. The card uses the onboard memory to quickly process textures through the GPU. The more memory you have, the more textures the processor can load, meaning you can use a bigger monitor on it. The increased capacity can also help stabilize your framerates since there are more textures readily available.

I have a ATI CrossFire Board, and I have always run one Nvidia Card.