The graphics card and PSU are not great. The IGP on the CPU is probably faster than an 8400GS. The thing is that you need to spend more money on the build if you want something that will last longer. For example even though your brother is gaming at a lower resolution investing in a decent graphics card will work out cheaper over a few years.
The following build is from Ebuyer.
http://i559.photobucket.com/albums/s...FTH/Build1.jpg
I would check on the Ebuyer forums whether the card will fit the case though!
Getting a cheaper CPU would help reduce the cost of the build. A Core i3 2100 would be great for many games and would be the best choice
IMHO at around £90 to £100 for running Matlab. However,a Phenom II X4 955BE or a Phenom II X4 840 would be faster than a Core i3 2100 in most video encoding programmes and will be competitive in games which can use four cores.