Here's what eVGA recommends for use with a GTS450: EVGA | Products
(400W, with 22A available at +12V). I'd expect your Tagan 480W PSU to be adequate, although you don't list its 12V spec. (That's usually listed on a label on the PSU itself, if you feel like looking in the PC's case.)
The motherboard requires a 24 pin ATX power connector, and a 4 pin +12V CPU connector. I can't recall whether your old Socket 939 system needed the same, but I think so. (In a pinch, you may be able to use a 20 pin ATX connector.) If the PSU lacks a 6 pin PCI-E power connector, a Molex adapter is usally supplied with the graphics card.