A:
It depends on the rest of the system. The Dynex 400w PSU, if you dissect it, is only rated for 250w continuous power. See this review:
http://www.hardocp.com/article/2008/05/13/getting_best_buy_on_power_supply/2#.U1kcY1VdXHo
If the rest of the system has anything more than basic components, then the answer might be no. Best Buy carries a Thermaltake 430w PSU that would be better, and is only $5 more:
http://www.bestbuy.com/site/430w-tr2-atx-power-supply/9827301.p?id=1218179698427&skuId=9827301&st=categoryid$abcat0507009&cp=1&lp=3
Alternatively, you could go for the Corsair taht is only a little more and is a really great PSU in terms of quality:
http://www.bestbuy.com/site/cx-series-modular-cx750m-750w-atx-power-supply/8324202.p?id=1218872214205&skuId=8324202&st=categoryid$abcat0507009&cp=1&lp=1
It will also allow further upgrades down the line, and would easily go into your next system.