Defintely try to work in some Opteron 265s. They are going for $307+shipping each at Monarch right now, so they are a steal compared to their old price of about $850 each when they first came out. A good dualie Tyan board should run you about $350 or less, so you should be able to get the processors and mobo for around $1000. That leaves about $2000 for the rest of your system and so long as you don't go crazy and blow the rest of your budget on a Quadro FX 4500 you should be fine with a $3k budget for the system only.
If the $3k figure includes a monitor, then it might be a bit tight trying to get dual 265s. If your friend really has his heart set on a 2405FPW, then you could always go for a single core opterons at first a dualie board at first and then upgrade to dual cores later when the prices drop more on the s940 parts. I think you might also be able to run a single 265 in a dualie board without major issues, but I'm not 100% certain. If you do decide to put together a dual dual-core Opteron system for your friend, it wouldn't hurt to spend some time over at 2CPU.com to better familarize yourself with all the little details that are involved with putting together a dualie rig.