The connection speed of dial-up services is limited to a (theoretical) maximum of 56Kbps (which is why you won't come across any dial-up modems which offer faster speeds). In practice, various technical restrictions mean that actual connection speeds are usually lower than this. (I'm using dial-up to send this message - connected at 42.67Kbps).
If you're going to be using dial-up, I suggest taking a look at the various magazines (or websites) which review ISPs and opt for a service with a decent 'contention ratio'. This tells you the number of users who may be sharing a connection to the internet. (A contention ratio of 20:1, or worse, should be regarded as poor. 10:1 is fairly good. 5:1 is very good but probably expensive).
I use Claranet as my main ISP (because of a decent contention ratio) but I sometimes connect via Webtribe. This latter, little known, ISP can be really useful if you can live with the fact that they only offer 'pay per minute', rather than 'flat rate', services. They offer unlimited webspace and, because you can sign up as many times as you like, unlimited e-mail addresses. If you want faster access, they allow you to use a compression service (similar to that offered by OnSpeed) completely free of charge. This, in effect, increases the dial-up speed by a factor of 5.
Links:
http://www.clara.co.uk/
http://www.webtribe.net/main.php
Chris