I did a check on the GTS 250, it requires 12 / 12.5A on the 12V rail at maximum utilization.
The Odin 585W PSU gives 16A on 12V1 and 18A on 12V2 (should be the PCIe one).
My cheapy 430W PSU was 14A on the PCIe 12V rail. Most power supplies should be sufficient for the GTS 250.
With regards to PCI2 2.0:
Any chipset after the P35 should be PCIe 2.0 compliant.
The card should technically work in a PCIe 1.0 slot, but the bandwidth is half that of PCIe 2.0, so some performance issues might occur.
If the motherboard is 3 years or younger (Core 2 technology), it will be PCIe 2.0. So no problems.