My 5870 is working great for me and well worth the cash back in December. Played more games on this one card then on my previous two Nvidia cards 8800gtx(retired)/260(in use)
. If a 5870 is not too much more then a 470 then I would recommend a 5870 otherwise a 470 would kick some as well. In other words you win either way as far as I can see. If you are more interested in 3d and physX then the 470. While ATi promisses 3d in the future it is not here yet. If you want to expand you work environment for more screen space and some 3d gaming, then a 5870.
Gigabit DSL, Q6600 @ 3.4ghz, Mushkin 4gb DDR 800 OCZ 2x2gb , PowerColor HD 5870. 24" Acer LCD, 1TB hd with Vista 64 Home Premium.
Foxconn 780G, Athlon64 X2 5600 @ 3ghz, Mushkin 4gb DDR 800 OCZ 2x2gb , eVga 260 GTX. 17" MagAcer LCD, 120gb Vista 32