RAM: Crucial 4GB (2x2GB) DDR3 1333MHz/PC3-10600 Ballistix Memory Kit CL7 (7-7-7-24) 1.65V £91 http://www.ebuyer.com/product/179278
You will have a hard time finding better RAM at any other sites, certainly for the price.
Sorry for being ~£40 over. To cut back a bit you could get a 500GB HDD and a motherboard that doesn't do crossfire. Obviously try and order from as few sites as possible, my recommended maximum would be from two sites.
As for dust some dust filters would probably help out a bit, and of course clean inside once every few months. I use a vacuum cleaner to get out the worst of the dust in my case. Obviously use a bit of caution when near sensitive components.