Unfortunately mate, your not going to get very far with a budget of £250. You may be able to get a base unit for general office work / downloading but don't be expecting to play games with the onboard GPU, but consider adding a reasonable one in at a later date when funds allow.
Do you need a copy of Windows 7? If your student you may be able to get it cheaper.
How about something along the lines of the following for the base unit.
YOUR BASKET
1 x
Cooler Master Elite 430 Windowed Case - Black (with 500w Elite Power PSU) £64.99
1 x
AMD Athlon II X3 Tri Core 450 3.20GHz (Socket AM3) - Retail £59.99
1 x
Asus M4A78LT-M 760G (Socket AM3) DDR3 Motherboard £49.99
1 x
Corsair XMS3 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (CMX4GX3M2A1600C9) £33.59
1 x
Western Digital Caviar Blue 500GB SATA 6Gb/s 16MB Cache - OEM (WD5000AAKX) £29.99
1 x
Sony Optiarc AD-5260S 24x DVD±RW SATA ReWriter (Black) - OEM £15.98
Total : £266.53 (includes shipping : £10.00).
Save up and get the graphics card later on. The 5450 won't play games so best to save up for something like at Radeon HD5830 which can be had for around the £80 mark. Paying a bit more for the Tri-Core over the dual core should "future-proof" the system a bit and you can add in a Phenom II X4 or X6 in a couple of years.
Worth shopping around and looking for special deals and perhaps consider second hand market too.
