1) The 6870 is a notch slower than a GTX470, and priced accordingly.
AMD suggests a 500w psu for it.http://www.amd.com/us/products/desktop/graphics/amd-radeon-hd-6000/hd-6870/Pages/amd-radeon-hd-6870-overview.aspx#3
2) The GTX470 is an older technology, and needs a bit more power, like 38a on a 550w psu. If your psu has 38a and is a quality unit, you might try it, but be prepared to change out the psu.
3) The GTX560ti has similar performance to the GTX470 , and needs 500w with 30a on the 12v rails plus two 6-pin PCI-E power leads.
4) For nvidia cards, I like EVGA; they seem to have good support.
5) Your 450w psu might work, and is worth a try. Graphics card vendors tend to suggest stronger than necessary psu's to account for poor quality units and to forestall returns because of marginal power. But you should be prepared to spend an extra $50-$70 for a quality psu.
5) I might suggest the EVGA GTX560ti superclocked. It is close to your budget with a rebate and free batman arc game:http://www.newegg.com/Product/Product.aspx?Item=N82E16814130610