Your 3870 is not a low end card. Do you want something with
the same level of performance (gaming) ? Or do you just
want something to test the set up with ?
There should be a few HD 5450s around, which would be a generation old
in terms of silicon. Xbitlabs measures cards like these as 3.2W idle,
and maybe 10W busy. That's why they have passive coolers on them.
Passive coolers are not always a great idea - they do avoid fan failures.
But I have had cards, where they needed a bit more cooling than passive
can provide. Still, for the price, this would be in the right
ball park of "cheap".
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150668
The video RAM bandwidth, might not be that much different
than my collection of FX5200 cards here. That's how low end they are.
And the power rating here, is a theoretical, rather than the
actual measurement of a single card. (They list 20W.)
http://www.gpureview.com/Radeon-HD-5450-card-625.html
The competition for that card, would be Geforce 210, and
it's roughly the same power loading.
http://www.xbitlabs.com/articles/graphics/display/gpu-power-consumption-2010_3.html
Paul