PhysX cards are, in my opinioin, really not worth buying. Save your £200 and buy some more RAM or better graphics cards or something.
From what I've read in PCZone and around the internet the card doesn't actually give you much more, and just creates more for your graphics card to render (ie particles and chunks of ground from the ground in GRAW).

Besides, DX10 isn't far away and it's going to basically be able to do what the PhysX card does for free. Apparently, it'll be able to take advantage of the unused power of your graphics card and use it to process physics. How successful this is or even whether it's entirely true, I'm not sure, but my advice would be a PhysX card just really isn't worth the small fortune that retailers are asking for it.

Of course, if you're sickeningly rich and already have an insane rig and just want something extra to boast about, why not?