If you want to make use of the two GPUs in a GTX 295 card, you want at least a dual-core processor. Also, for best transfer speed between the CPU and the card, you want PCI Express 2.0. The GTX 295 can draw up to 289W, which means that it requires both a 6-pin and an 8-pin PCI-Express power connection. Many power supplies lack the 8-pin connector, so typically a 2x6 to 8-pin adapter is included with the GTX 295. So you will want a power supply that is at least 650W with 6-pin + 8-pin connectors or three 6-pin connectors.
Also, as was pointed out, the card is wide enough to occupy two slots, so make sure there is enough room in the case for that.