One thing to bear in mind is that it will use more of the of the Photobucket monthly bandwidth usage allowed for the free account. However, I think you'd have to be using the account for alot more than showing photos here before the limit is reached. Perhaps worth looking into every once in a while nonetheless. Open Photobucket and click the 'account options' link at the top-right of the page. See the 'Account Dashboard'
Typical file size of a 320 pixel photo = 75 kB
Typical file size of a 640 pixel photo = 175 kB
When someone views your photo for the fist time, it will typically use up, from your account, the number of kilobytes shown above.
Photobucket gives the option of resizing to 640 pixels as the next size up from 320 pixels and that's why the new size was chosen. Most of us here use Photobucket. However, you don't have to go up to that size. Photos can be be less than 640 pixels if it's thought sufficient for the photo content. File sizes can often be made less if you resize photos in an editing program before
uploading them to Photobucket. Photo sizes can be varied to something below 640 pixels if desired and that would mean less file size and faster loading times. There are more options available in a photo editing program than are offered by Photobucket.
I've sometimes used 550 pixel photos in the 'larger photo' links, which I always feel is a good forum size. Photobucket don't offer that one though. Anyway, if you're into resizing, sharpening, cropping etc., use an editing program. If not, then just use the 640 pixel size offered as an option in Photobucket.
If everyone is happy with the new size and it stays, I don't think I'll ever have to edit another photo size, will I?