Here are some gaming benchmarks:
http://www.gen-x-pc.com/IntelvsAMD.htm
If a majority is not going to be for gaming, the Intel would probably be your best bet. The issue is the heat will be considerably more of a factor since it is a Prescott core. Also you mention the 2MB of L2 Cache. I don't have any exact quotes or anything but if I remember correctly, the performance impact of a higher L2 Cache is not very noticable. If your looking for an equivalent though, it probably shouldn't be the Athlon 64 3200+ but the 3500+ instead.
Dual-Core would have a miniscule increase in performance in general (Especially in gaming). Some applications do take advantage of Dual-Core already but they are very few and far between.