out of the 3 cpu's you have listed, the 3700+ 939 pin 2.2 ghz (watch out you dont get the 754 pin version which runs at 2.4ghz by mistake) would be best as it has 1mb L2 cache, the other 2 only have 512kb L2 cache.
the 3200+ is good for overclocking (dont know about 3500+) and can use money left towards more memory.
the opteron's are excellent chips. the 939 pin versions are selling over market price on ebay but still only 1/3 the cost of a true FX chip. the 142 is rated as same FX53 and the 144 = FX55 as reported else where on the net.
the only problem you may have is that AMD have found out what is happening with the opterons (people buying them instead of FX's) and have clamped down on there sale to third parties to try and make sure they only end up in server systems.