You already have a 5750? or thats what your thinking of getting? If youve already got one, get another a go CF, if not get a 5770 for around that price.
As for workstation gfx, while they may give you a small noticable difference in workstation like applications, they wont do to well at all playing the latest games.
What resolution does your monitor run at? That's the #1 deciding factor when trying to decide what GPU to get for gaming. If you don't need Crossfire support then you could save quite a bit by going with a cheaper motherboard.
Is crossfire important to you though? With even a 5870, going from a 8x speed to 4x speed, youll notice around half the performance on 4x compared to 8x... you can find out more on the p55 scaling article http://www.tomshardware.com/reviews/p55-crossfire-nf200,2537.html
In that article, look at the benchmarks for more info comparing 16x/8x/4x speeds to one another