Well I can't tell you about the 3700+, but I do own a 4000+ and a 3800+ (X2). I initially setup my gaming system with the 4000+, but since I had started to do more transcoding, I felt dual core was needed. Here are my numbers from 3DMARK05:
3800+(X2) @ 2.4ghz / 7800gtx 256mb(490/13100) = 8635
4000+ @ 2.7ghz / 7800gtx 256mb(490/13100) = 8589
Then I went SLI!!!
3800+(X2) @ 2.37ghz/ 7800gtx 256mb(490/13100) SLI = 12047
4000+ @ 2.64ghz / 7800gtx 256mb(490/13100) SLI = 12249
My memory is Corsair XMS Twinx2048-3200c2pt 2-3-3-6-1T
There's really not a big difference in the numbers when I compared the 2, but for me
I felt the 3800+ was smoother although they performed about the same in my setup. Hope this helps...