A 450W PSU is more than enough for that computer. If I was buying one I'd get this http://www.newegg.com/Product/Product.aspx?Item=N82E16817139026&Tpk=corsair%20430
Proof that it is enough power for the computer.http://extreme.outervision.com/psucalculatorlite.jsp
There isn't really a disadvantage getting a 500w since it would allow you more upgrade options in the future (although those are mainly GPU upgrades and since you only want to do photo and video editing it really isn't needed). The difference in power consumption at only 50W difference would be almost insignificant only difference would be initial price.
Since your only using photoshop that GPU you picked should be fine but if you want one slightly better you can pick up a 7750 for only about $10 more and it should be a bit better for photoshop and other video editing software.
For windows 7 you only get the OEM version this includes one install for a computer without any operating system on it. The retail version has three upgrades on it so you need an older version of windows on it already.
Hope that cleared up your questions.