are you saying that there is no need for the 5750 then? I suppose if nothing else it will be more future proof.
Future proofing doesn't exist.
There is new generation of cards coming out every about 6-8months now.
Look at how everyone was future proofing their setups just around 8months ago with the newest tech 4870 and 4890 yet 3-4months ago, now it's all outdated and easily replaced by even 5770, not to mention 5850.
Buy what you need now, not what you might need later.
Also another option is to consider the 1 model up from that mobo for 10quid more as it has CF support, so you could buy one cheap card now, see how that works for you, and if you feel like needing more, add another one in CF later for silly money.
I'd try this one:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-138-XF&groupid=701&catid=56&subcat=1274
Might sound silly but you'll get suprised how much technology has moved on.
This is probably somewhere around 3850/3870 performance which just around a year ago was a top notch card and can still do fine in most games at mid res.
And if it's not good enough for you, then I'll personally buy it off you as I want one for testing anyways ; ).
I'd say it's worth a try. I know from my long experience in pc building that many people don't even realise how much more powerfull PCs are than they were few yrs ago.
It's amazing how nowadays you can get an amazing build for just 400-500quid where few years ago it costed at least 800-1000 to get minimum spec.
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http://www.*****.net/content/item.php?item=22280&page=6
interesting review showing the low end cards, as you can see, without much eyecandy they can cope just fine and cost a scratch of the price.