the 550BE isn't a bad cpu at all, you just need the right motherboard to unlock the extra two cores (if you're lucky). But this is WAY overpriced. I was looking into making a build with similar parts (550BE, 4890 Vapor-X 1GB, OCZ Obsidian DDR3 4GB, Gigabyte GA-MA770T, OCZ ModXStream 700w, Coolermaster HAF 922 case, and CoolerMaster V8 cpu cooler) and that was around $630 USD (I don't know pounds or euros prices).
I would HIGHLY suggest building your own computer and just paying someone to put it all together for you. That would save you TONS of money and get you way more bang for the buck.
Look at a lot of reviews for parts you want, 3DGameman on youtube, TigerDirectBlog on youtube, PCWizKid on youtube, and overclockersclub.com
P.S. A 4870 would be able to run all those games on high, if not max (maybe excluding MW2, I know a GTX285 gets around 110-120fps in mw2 fully maxed)
Edit: Here's a benchmark for MW2 with all the latest graphics cards - http://www.pcgameshardware.com/aid,699312/Modern-Warfare-2-Graphics-card-benchmarks-of-the-latest-Call-of-Duty/Practice/
A 4870 512MB should get around 75 fps at max settings (4xAA and 16xAF) at 1280x1024 resolution