I am in the same position as the OP.
I want to replace my aging SN25P (X2 939) and have a budget of £600.
Light gaming and needs to run 3 screens.
I am waiting for the latest Sandy Bridge Shuttles to become available before splashing out the cash.
shuttle.eu/products/mini-pc/sh67h7/overview/
shuttle.eu/products/mini-pc/sh67h3/overview/
Barebones will be about £250. However for CPU, RAM, HDD and GPU, you might have to go for £300 to get those.
My advice, wait for when one of these is out (hopefully in next 4 weeks and on offer on scan )