This is a great suggestion, but with that 450w Corsair PSU you wouldn't have very much overclocking headroom.
That PSU would get torn to shreds if you overclocked very much. (Figuratively speaking.)
Not to mention the 450w only has a single 6 pin connector while the 5870 needs two.
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.
well, it seems you have decided against it, but I like the i7 build. The only thing with that one is the psu in it isn't good enough for crossfire in the future if you wanted to do that.