Most 3400+,'s (at least the ones AMD recognizes exist) are socket 754 which won't be venice core compatible. There is also a 939 3400+ ( sometimes available with a mobo purchase from places such as tigerdirect ). Any 939 board will (should) be able to handle the new venice cores with a bios upgrade.
Your best bet would probably be getting a 3500+ winchester now (though some might say a 3000 or 3200). I think that by getting a winchester, it will have more selling power after you upgrade.
unless you get a terrific deal make sure the cpu is a winchester that way when you replace it, it will be worth more to help you pay for the upgrade to venice.
Today, also make sure the motherboard has PCI-Express (PCI-E) whether or not you get an SLI board (which would allow two PCI-E graphics card slipped together) is up to you. I like ASUS such as the A8V-E (the -E signifies PCI-E where the regular would just be A8V and have AGP, both may say deluxe) based on a via chipset or the A8N SLI based on the NForce 4 chipset. (there is also an A8N SLI-Deluxe that offers dual gigabit lan ports and the ability to support more hard drives)
Many will swear by gigabyte or msi
If you go with a N Force board, stick with an NVidia graphics card such as a 6600 or 6800... since you should be geting a PCI-E motherboard, make sure to get a PCI-E video card... my understanding is that only NVIDIA graphics cards allow you to use the sli features of an NFORCE 4 SLI ... But I personally like ATI cards better.