Ok here are the requirements:
ATI Radeon™ HD 4850 System Requirements
* PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
* 450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
* Certified power supplies are recommended. Refer to Certified Power Supplies
for a list of Certified products
* Minimum 1GB of system memory
* Installation software requires CD-ROM drive
* DVD playback requires DVD drive
* Blu-ray™ playback requires Blu-ray drive
* For a complete ATI CrossFireX™ system, a second ATI Radeon™ HD 4850 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required
this should be plenty for you: 71 pounds including VAT.
Corsair 650W TX Series PSU - 120mm Fan, 80+% Efficiency, Single +12V Rail - Ebuyer
I assume you are getting a i5 or 1156 i7 with that motherboard.