Just playing devils advocate here ...
I'm assuming you are using the standard choice of GD or ImageMagik, in which case image quality shouldn't be a problem. Especially as we are talking about creating images for display on a web page, at 80dpi. The standard thumbs you already create look great.
And as you seem to be doing the resampling into the three standard thumb sizes during the upload phase (I just peeked at the code), you could do those from the original large image, before resizing the uploaded file prior to saving it, so you don't have "resampling resampled images" issues.
As for memory ... if a site doesn't have enough memory, they wouldn't have to use this feature. Personally I'm setting up shop on a machine with 4GB, so I can afford to spend a bit of memory to make my users lives easier.
One of the main reasons I'm suggesting this, apart from user convenience, is actually for tech support reasons. One of the most common questions we've gotten in the past when we've provided upload features like this is "How do I shrink my picture so I can upload it". Being able to cut out 90% of those support calls would save a significant amount of money - each support call averages about $25 in total "cost of doing business".
Just my $0.25. Or $25.00.
-- hugh