Yes Rocketfish is a best buy store brand and yes there is a large profit margin on these which is likely why they pushed you to buy it. (As for bestbuy's offerings; the antec Earthwatts 650w would have been a much better and slightly cheaper choice in the same price range but an even cheaper 450 or 500w PSU would have been sufficent for your system)
Here's what Hard OCP's testing found out about the Rocketfish PSU (It isn't good The same revied also tests the best buy store branded 400w Dynex PSU with even more dissapointing results)
http://www.hardocp.com/article.html?art=MTQ5MSw1LCxoZW50aHVzaWFzdA==
The actual OEM is a company called Huntkey and not only is it a rebranded huntkey 650w and fails to deliver 700w the build quality is also poor. The Hard OCP test found that the PSU was actually a marginal 550w unit and is defently overrated. My personal reccomendation is to return that overpriced piece of junk if you are still in the return period.
I have put a geforce 8400 in a dell insperon 530 with the stock psu and it is perfectly sufficent as long as the system is an otherwise stock configuration (not alot of other drives,expansion cards, etc.) I believe dell offered the 8500gt as a factory option in that system and is shipped with the same bestec 300w PSU as the base units. That being said the bestec 300w is only mid range in quality and is near it's max with that card so a 450w or 500w PSU would not be a bad upgrade. And try to stick with the following brands Antec, Corsair, Enermax, FSP, PC Power & Cooling, Seasonic
Here are some reccomendations for your system:
FSP 500w $70 (They have a 450 w for $55 but it has only 2 SATA connectors and the Insperon 530 is an all SATA system so you will want 4 sata connectors unless you plan on useing adaptors, the 500w has 4 SATA connectors which is the number of drive bays in the 530)
Antec EA500 500w $70
PC Power & Cooling silencer 500w $70
If you don't want to shop online computer shops generally offer better prices on PSU's than best buy. Just be careful to get the major brands mentioned earlier since computer shops also tend to stock some low quality overrated junk.