That's too subjective, because it depends on the game and resolution you play.
Looking at your specs, you are using a 1280x1024 monitor:
http://www.ncix.com/products/index.p...acture=Samsung
For this resolution, you are probably going to be OK with a $130 5770 or even a $280 5850 is better value than GTX470. To try to answer your question, look at the lowest resolution graphs here:
http://www.anandtech.com/video/showdoc.aspx?i=3783&p=9
1680x1050 GTX470 vs. 8800GT
Crysis = 44.1
(2.7x) vs. 16.6
Battleforge = 69.2
(3.0x) vs. 23.1
HAWX = 124
(3.0x) vs. 41
L4D = 130.6
(1.7x) vs. 76.7
Mass Effect 2 = 77
(2.0x) vs. 38.8
Wolfenstein = 99.3
(2.1x) vs. 46.2
If you still plan on sticking around on a 1280x1024 monitor, I would strongly consider upgrading that first to at least 1920x1080. Remember these test are done on a faster overclocked Core i7 @ 3.33ghz which is going to be faster than your dual core at such a low resolution. So if anything, at 1280x1024, you'll get even less of a boost.
Honestly, I wouldn't get anything faster than 5850 and I don't see any reason to spend $70 extra over $280 5850 for a GTX470. If you want NV only, I'd consider getting a used GTX260/275 instead.