Noone else seems to have mentioned this,
1. overclocking that cpu on the 450w? questionable for longterm stability
2. crossfire on that psu? I wouldn't if I were you...no,really I wouldn't
and,
3. probably the biggest issue you have is that that mobo only supports 16/4 crossfire, you will most likely find that your performance decreases, not increases,
Sorry to bear the bearer of bad news/crossfire dreams stomper, but you would be better off with a better single card (and beefier psu once funds permit)
just reread and Shane did actually point out the x4 lane issue
I had a similar desire some time back,
http://www.tomshardware.co.uk/forum/291669-10-crossfire-5770
but without a more powerful psu I couldn't test it,
Moto