I have a simple question or 2 for you. Would you consider self building the entire machine? This will get you more for your money as you can splash out on a graphic design / animation card rather than something aimed for games. However do you play games?
However do consider also looking on
I have seen many a 21inch CRT screen with a total cost of collecting it. CRT I still feel is better than LCD and also if it is free you can look at £700 tower units and get even more grunt.
So can you be driven to building it yourself? And do you need games ability?