I really would have asked, first off, if you need one at all, as in high end card. What level of cad work do you do, are you doing such intensive stuff that you're waiting hours on end for something to finish rendering workwise, or well how important is it.
It strikes me that if you've been using a 3870 huge Cad performance isn't a huge priority, and likewise if gaming is only occasional, maybe something much cheaper would be the better option?
It completely depends on where you feel your card is too slow right now to be honest.
If its a case of Cad isn't gpu limited for you, and its gaming thats a tad to slow these days, something midrange might suit you very well, or something with EOL pricing.
A 470gtx, ok it uses more power, not that you'd realistically notice, you could probably save yourself £100 and get a 470gtx over a 570gtx and never actually notice the speed difference in the slightest.
Again if Cad isn't really that important and its mostly gaming, you could look at some of the silly priced 5850's/5870's going at the moment. For gaming the 5850 is just a ridiculously large upgrade from a 3870, and £125 I think at the moment with a basically silent cooler and the ability for a 40% overclock potentially(30% is easy). Performance wise in terms of value a oc'd 5850, a oc'd 470gtx, will both match a 570gtx, and aren't far behind at stock, both because they are on end of line pricing, are insanely good value.
Honestly right now I wouldn't recommend anything high end and new, 6950/6970/570gtx/580gtx performance/£ they are demolished by 470/5870/5850, because there are only a few of those cards left so they are on massive sale pricing.
If you're coping with a 3870, I really get the feeling you won't "need" a 570gtx's performance so theres a lot better value from the older parts which are only 10-15% slower.
Obviously if you do need the performance and its your work, and your waiting on stuff to finish in Cad, it can easily be worth the extra money, if not while the old cards are in stock you can be saving yourself £100-150 for essentially the same level of performance.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-066-GI&groupid=701&catid=56&subcat=1810
470gtx, not As good value, loads of these were going for £165 a couple months ago, at current pricing the extra £10 for a vastly superior cooler, overclock and lower noise is WELL worth it.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-103-MS&groupid=701&catid=56&subcat=411
5850, £125, great cooler, quiet, very VERY good card, overclocked it will easily beat a 470/570gtx, they overclock like crazy, mine is sitting here right now from 725mhz stock to 1050Mhz.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-147-XF&groupid=701&catid=56&subcat=1502
5870 £190 great card, but its only 10% faster clock for clock than the 5850, which is almost 35% cheaper and with a better cooler.