im looking for a £900 gaming pc build around the nvidia gtx 780(no water cooling)

you can start from this build :)
i5 4670K
2x 4GB ram
the best GTX 780 from Gigabyte GHZ edition

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor (£161.99 @ Aria PC)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler (£25.20 @ Aria PC)
Motherboard: ASRock Z87 Extreme4 ATX LGA1150 Motherboard (£111.93 @ Amazon UK)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory (£64.04 @ Amazon UK)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£43.90 @ CCL Computers)
Video Card: Gigabyte GeForce GTX 780 3GB Video Card (£399.99 @ Amazon UK)
Case: Cooler Master K380 ATX Mid Tower Case (£36.59 @ Ebuyer)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply (£45.00 @ Aria PC)
Total: £888.64
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-02-16 01:38 GMT+0000)
 

This build will definitely last you 3 years before you need to upgrade. If you want you can even SLI two 780s for maximum performance!