My current system is a AMD 4800 X2, Geforce 7950 GT and bioshock works well but does suffer slowdowns at certain points and the same can be said for company of heroes. I'm redecorating and finding space for massive PC isnt easy, an iMac would fit in so much better and I have been wanting to try OSX for years now.
I am certain that there is probably slow down in CoH, but it is definitely not noticeable, I'm loving this game right now, been playing it pretty heavy the past week.