That PSU would get torn to shreds if you overclocked very much.
"450W" PSU would, but not this one.
Also, the 5870 has really good performance per watt. It will use 188W under full load, and less than 30W idle.
The system in total (including CPU) will probably use about 350W total under full load (CPU + GPU)
Well below the PSU's ability to continuously supply at least 450W
A crap PSU rated at 450W might only achieve 450W peak under ideal conditions (i.e. not the conditions you'd use a gaming PC in), so one of those might have problems with this build using about 350W. The corsair VX-450 is miles ahead of a crap 450W PSU.