What's your power supply? And I'd go for a 7850 over a 6950. If you get the 7850, then whatever PSU you use with a GTX 260 will be plenty. In the U.S., that would run you $250.
What websites do you use in the UK? Pretty much any SATA III SSD will do fine. I'd go with the cheapest I could find (although not the OCZ Octane or Petrol lines). I suggest a 120GB version. In the U.S., that would run you $120.
Other than that, your system should be plenty for any task today. Although you may want to consider picking up a third 2GB DDR3 RAM stick. You'll want to match the specs on the two you already have (such as 1600MHz CL9 1.65V).
Lastly, you may want to look up some info/start a thread on overclocking that i7-920. That CPU is no slouch, but it will probably OC to 3.2GHz with no voltage adjustments or additional cooling. If you overclock, be careful with your temps. Pick up a $10 Corsair A50 cooler w/ promo code EMCNGHN63: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
From the fact the OP is using the euro sign I/m guessing that they won't be able to buy from newegg (US only).
For 400 euro you should wait for 1 more day when Nvidia will release their new GPU, this will play with the pricing of AMD cards and may force some price drops. However as prices currently stand the 7950 comes in just shy of your budget: http://www.ebay.co.uk/itm/XFX-Radeon-HD-7950-3GB-Core-E...
If you're looking to get both a gpu and an ssd for 400 euro's then here's my recommendations: