Well in May, PhysX cards are supposed to be coming out at around 300 bucks. For a workstation, i would defintly go with dual core and since you have a 2500 dollar budget, go for the AMD 4800 X2 S939.
For your motherboard, make sure it has a pci-ex16 slot or two if your going to SLI or crossfire. I would go with a 7800 series or 7900 series card for video. Maybe two 300 gig SATA. defintly get a good psu, and make sure the pin number matches up with the mobo (20/24/20+4).
EDIT: read this....