That's a terrible gaming rig besiktas1, the £400 machine I specced for instance will demolish it in games so it seems a bit hard to justify spending £900 on it!!
Given your tasks you'll be wanting a quad core but if you're gaming then Dell is going to offer terrible value. Any real reason you can't build your own rig? My £600 config looks ideal for you, heck you might as well bump it upto 8GB RAM, while you're at it.
If you won't go the "build your own" route then I recommend YoYo Tech, this PC looks ideal for you:
http://www.yoyotech.co.uk/pc_system.php?pCid=16&pcSysId=169
Do the £5 upgrade to a 4870 512MB and I'd recommend upgrading the PSU to the Coolermaster for £20 for some piece of mind, still less than £640 even after them upgrades.
Heck you could get one of these watercooled, pre overclocked monsters with a BD drive and a top of the line 4890 1GB GPU in your budget:
http://www.yoyotech.co.uk/yoyotech-...-reader-onboard-sound-software-p-2300846.html
or
http://www.yoyotech.co.uk/yoyotech-...sound-vista-home-premium-64bit-p-2300847.html
The big problem with going with something from Dell is they're terrible for upgrading. The motherboard is locked down, the cases crappy and the PSUs, while servicable, generally need replacing if you're to upgrade the GPU. If you get a custom build, either premade or built at home you get to choose your parts and will be able to keep stuff like the case, PSU and motherboard through many upgrades and overhauls. When people get a Dell they tend to junk it all in one which is just a bad use of funds, you're money works much better for you if you do gradual upgrades over time to enhance its lifespan.
Having solid, user replaceable/servicable components means you'll never have to buy a whole new system again, upgrades over time well let your system last indefinitely.