The 450W SmartPower 2.0 is at the lower end of the scale for Antec's PSUs and might cause problems depending on the rest of your system specs.
It has dual +12V lines giving a total of 32A, but your X1950XT will eat up most of these amps (ATI recommends +12V=30A minimum). If you start to notice 3D games crashing, check the PSU voltages and system temperatures using SensorsView and you'll probably find the PSU can't handle the stress.
And to answer your original question, the X1950XT is one of the best in its price range. I nearly bought one recently but decided to go for the X1950 Pro (Sapphire Ultimate version with extra performance and better cooling system).
Since that came with the case, you could always sell it. But personally, if I were in that situation, I would get the 650 w and keep the 450 w around for troubleshooting. I always keep my old parts around in case I need them.
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