On the assumption you're not scanning for archive, but simply going for "good enough to see/use: colour photos scan well at around 175dpi. You don't gain much more by going higher. Negatives scan well around 2000dpi, you don't get much extra benefit by going to 4000dpi. These would give you the sort of results you would expect from a 6 megapixel camera (very, very approximately).
After that, you might want to use a picture viewer such as Irfanview that will let you rescale images to the size you want to post.
If you're scanning and saving directly as jpeg/jpg, you can choose the "quality" level somewhere in the software. Jpeg is a "lossy" compression system: each time you edit the same picture, unless you are careful, you will lose quality and the picture will get smaller when you save it. With quality=100, the jpegs will be pretty big. 100 is rarely used. Quality=90 is a good place to start, and for posting to the internet you can go down to quality=75 or 60 without too much fuss, and the pictures will be way smaller.
I used to scan to TIFF, and now I scan to DNG. These are lossless formats: you don't lose quality when you edit and save them. Once I've scanned my original file, I simply use Irfanview to create a jpeg of the right size and quality for any particular purpose. The logic is, you can always go down in size and quality to get the final product, but you can't go up: a bad scan is a bad scan no matter what you try to do with it.
The 3200 is a nice choice, by the way.