to find out what the mobo is, download a program called CPUZ.
as for the PSU, its almost certainly going to be auful. a friend of mine is in a similar situation, with a similar computer, and the PSU was a 250W no name.
when you get home, i would also suggest checking how long a graphics card you can fit in your case. i believe the 6950 is 275mm long. there may be a way of shifting things around in your case to get you more space, like moving a hard drive cage or hard drive. i had to move a few things to get everything to fit in my case
also, you will need to measure the gap between the motherboard and side panel of the case, because most good CPU coolers are 160mm tall or more. (once again, this was a problem for me, because my case only has room for 155mm tall CPU coolers)
since you have £400 to spend, you can get a great graphics card, and a nice PSU to boot. something like this will be more than enough:
YOUR BASKET
1 x
MSI ATI Radeon HD 6950 2048MB GDDR5 PCI-Express Graphics Card £199.99
1 x
BeQuiet Pure Power L7 530W Power Supply £42.98
1 x
Corsair A50 High-Performance CPU Cooler (Socket AM2/AM3/LGA775/LGA1155/LGA1156/LGA1366) £24.98 only if motherboard is P series
Total : £279.96 (includes shipping : £10.00).
*edit*
looks like i was beaten to it for a lot of this info
