My gut instinct is that that is a good price. PCs are like Alfa Romeos when it comes to depreciation, whereas Macs are like German cars, they depreciate slowly (which also explains why they are typically stolen in preference to PCs).
If you are going into photography, then a Mac is almost certainly a better choice. I have used a mixture of Windows, Mac and Linux in my job. 4GB of RAM is plenty, and I have that nVidia GPU in my own laptop, and it copes fine with my multi-core research (just make sure it is the 512MB version, not the 256MB). First thing I would do though is bin Vista, and swap it for XP if you still need Windows. Vista can do some funny things to your hard drive partitions, and could compromise your OS X installation if you are not careful.