I'm probably not going over 6gb (I've ben using 2gb for 2 yrs now). Is that the only thing the UD4P has over the UD3R, ram upgrade? It's not a whole new cooler design or something?
House is ~2000 sq ft, two floors.
The router will work fine. And yeah, the only important difference here would be that the UD3R only has 4 RAM slots, so going over 6GB would slow you down as they wouldn't be triple channel anymore. The 5870 should be significantly faster and push prices down on everything else a good deal, with a less than a month before they come out, you might want to wait.