The simplest way to move them would just be highlight them all, and drag and drop to your flash drive, but I'm going to bet that 1800 photos won't fit on a 2 GB stick if they're taken at a fairly high resolution.
You may have to copy them over in groups.... a few hundred at a time. That, or run out to MicroCenter and pick up a bigger flash drive. You can grab a 16Gb model for $40 (maybe cheaper if they're on sale).
I'm not sure what you mean by "won't convert" unless you just mean that they "won't copy". Nearly all photo formats are easily recognizable on both PC and MAC.
Another way to do this would be to highlight all your photos in iPhoto and use the File>Export option to be able to specify what file type you want them exported in (jpg, tiff, or png) and you can select a resolution to scale them down to in the process. This would get them to all fit on a 2GB flash drive with a little bit of math, but you'd be scaling down the photo quality as well.
One thing I love about a MAC to MAC upgrade is the ability to hook up a firewire cable and import all of your profile settings, applications, and files to the new MAC upon setup. I'm not sure if this can be done easily from MAC to PC, but a direct firewire connection is about as fast as it gets for moving large amounts of fires around.