The OCZ StealthXStream 600W is a quality (although rather old) PSU from FSP that is able to run even an ATI5850 or your GTX280 easily. However, in my opinion, it is not the proper unit for handling a GTX295. Its maximum combinated power for the +12V, +3,3V and +5V is 580W, but as Alex said, its power distribution is not the best possible. Only the +12V4 (18A) provides energy to the two PCIe connectors. Other two rails are for the CPU and the last one is for the peripherals. It is far for being enough.
If you want to do the upgrade, I recommend you to go for a quality 650W PSU which has not less than two PCIe connectors (one of them with 8-pin).
The ANTEC TruePower New 650W is a very good choice.
It presents a high efficiency (80 Plus Bronze certified), very good voltage regulation and ripple and noise supression. It is made by Seasonic.
It comes with semi-modular (hybrid) design (it presents some cables permanently attached to the PSU). It will handle your high-end graphic card without any problem. It has four (6+2)-pin PCIe connectors. They get the power from two 25A rails (all for themself). There are four independently regulated +12V rails rated at 22A (two of them) and 25A (the other two) which are over current protected. The max. combined +12V rating is 54A (648W).
All electrolytic capacitors are Japanese from Chemi-Con (in the primary there is one capacitor which is labeled at 85º C while the caps in the secondary are labeled at 105ºC). In the DC-DC converter they have used solid aluminum capacitors (this PSU is based on a DC-DC converter on the secondary which is in charging of converting the +12 V output into +5 V and +3,3 V).
Finally, It has five years warranty from Antec.