If you are not in a great rush, I'd wait for the clients to stabilise a little yet.
At the moment, the NVidia client is the most productive, but I suspect that things will balance out with the ATI client in the near future. This will give you a better choice for the best productivity per £, possibly adding ATI 38xx cards to the mix. I would expect some developments in the coming weeks, possibly a month.
I'm also expecting that a number of the NVidia cards are going to reduce in price significantly when the full impact of the ATI 48xx cards and the new NVidia 9800 GTX+ cards are fully available in the next few weeks. The Asus reductions are just an early indication of these changes, other vendors are going to have to follow suit, giving you more choice again.
I'm certainly waiting before I invest in my next upgrade to my farm. A few weeks could make a significant difference in how this change will work out and how much it will cost.