One minor warning: it may be difficult to undo such an upgrade, should you ever wish to.
The graphics cards that I've taken apart used thick thermal pads between the VRAM and the stock heat sink, rather than heatsink compound (thermal paste).
Some aftermarket coolers use adhesive to bond the VRAM and other individual heatsinks to the chips. I don't know how easy that is to remove.
I don't know about cost/performance, but Arctic Cooling products seem to be highly regarded: ARCTIC
(The page is for coolers that are supposed to fit a GTX 470.)