From Xbit:
http://www.xbitlabs.com/news/cpu/display/20050421034233.html

That's good news for current socket 939 owners, most will be able to use the x2 chips with a bios update. Intel dual cores require completely new motherboards. This also will really help amd win new customers, in my opinion. One of the main reasons people get intel p4 cpus is for mulitasking, but now with the amd dual core athlon x2, you get athlon gaming performance and dual core multitasking performance all in one.

Prices x2 4200+ 2.2ghz 1mb $537
x2 4400+ 2.2ghz 2mb $581
x2 4600+ 2.4ghz 1mb $803
x2 4800+ 2.4ghz 2mb $1001

At first the prices seem a little steep, but consider that your actually getting 2 processors for the price. 2 athlon 64 2.2ghz(3500+) cpus would cost $544, inline with the dual 2.2ghz 4200+ x2's price of $537.

Intel's dual cores are called "Pentium D" and will be available in the following speeds and prices:

-Pentium D 820 2.8ghz $241
830 3.0ghz $316
840 3.2ghz $530

http://endian.net/details.asp?tag=Smithfield

The pentium d is slower than the athlon x2 though, the highest pentium d, the 840 3.2, should be comparable to the lowest athlon x2, the 2.2ghz 4200+(which is actually like two 3500+ a64's), athough it still may beat it.
This is just my speculation though, actual performance may vary.

Compare the 3500+ to the 3.2ghz E here( http://216.92.52.205/index.html )
in your favorite benches to see a comparison. In many, the 3500+ comes out on top.