I doubt the HD3870 would work with only 15 amps on the 12v rail. Take a look at some low end 5 series card, they should have low power consumptions.
hi but 5series cards are crap aint they
The high end ones are really good. Any low end card is crap. The problem is that your PSU is crap too, it can't really handle anything more than your average low end card.
Your PSU will hold you back big time if you realy want a decent card.
Id suggest this...
http://www.ebuyer.com/product/172950
I dont think your PSU will cope with anything higher....might not even cope with that :/
again you didnt realy mention what you intend to play with the card or total budget.
The supply is working perfectly on my self-built system, with an overclocked E5300 at 3.8GHz and an overclocked HD4650.
Right this should do you fine.
http://www.ebuyer.com/product/152505
Reviews=
You could step up to a 500W model of that power supply for £45 if you wanted but that 400w should be fine on your setup if you had a 4650.
and
http://www.ebuyer.com/product/172950