since noone's replied at all I will suggest this: I dont think there's much demand for an 56k modem simulator, but there may be some train spotters out there who disagree. The only way i can think that you would be able to get a suggestion of what it's like would be to look at the size of the files that are being downloaded: if you're dealing with images and text and what not then just have a look at the size of the files and know that if it's anything over, ohh i dunno, 100k then people probably arent going to enjoy looking at the site. Image editing packages will tell you approximate download times at differnet speeds etc too.
I guess it's pretty much the same for the rest really - if it's a big flash animation then it's going to take longer, and if it's video or something then you shouldnt set your broadcaster to be outputting at much over 10kbps.
Alternatively, (and again if it's just images etc) you could resize them in psp or adobe and make them into tiff's and then progressively make them larger and larger images (resizing them up) until they're ~16 times larger than they are normally. that would be hugely inaccurate, but it may give you a better idea of how long they take to download (ie if you have a 250k jpg, turn it into a tiff and then make it so that it's ~4meg and then download it.)
That;s probably no good to you whatsoever, but it;s the best that i could come up with.