I wouldnt imagine it to be much FASTER . . . I would imagine that the 3500+ would be more flexible and more upgradeful in the future, like, Socket 939 now has motherboards that are both AGP and PCI-Express, while 754 is still stuck with AGP.
The 3500+ is more future-proof . . . if that's worth $150 to you, then I say get the 3500+. If not, 3400+ is the same quality.
AMD Athlon 64 3000+ Winchester @ 2000MHz
MSI K8N Neo4 nForce4 Ultra
ThermalTake Tsunami Dream Black w/ Side Window
Antec Neopower 480W
Maxtor 250GB SATA + Seagate 200GB SATA
Patriot 2GB (2x1024) PC3200
NEC ND-3520A DVD+/-RW
Dell 2005FPw 20.1\" Widescreen LCD
ATi Radeon x800XT
Avatar - Infiniti G35