Ok, now I understand what you meant by "using proxy servers to speed up my connection". I thought you expected to get the speed gain from a standard free caching proxy. Obviously you were talking about a compression proxy.

Well, there are several proxies with compression features, but most of them just provide the source code for a do-it-yourself proxy server. You have to realize that compression is a CPU intensive task and therefore you will hardly get it for free. But if you know someone who has a broadband connection, the required knowledge to set up a proxy server on his machine, and the willing to let you use it for free, you may try one of the following proxies:

http://khelekore.org/rabbit/
http://ziproxy.sourceforge.net/
http://resume.technoplaza.net/c/zrpc.php

As you pointed out, another option is to buy an ISP with some kind of dial-up acceleration, or a speeding service that works on top of your existing ISP. Those services claim varying degrees of speed increases (up to 15 times faster dial-up connections, which I find hard to believe). And they are not free, of course. I think you should read other user's comments and reviews about these services before buying anything. Here are some of the most well-known ones:

http://www.netzero.net/
http://portal.onspeed.com/
http://www.earthlink.net/dialup/

I have also found a free service based on server-side data compression and a client-side decompressing proxy. You have to install a Java application and configure your dial-up connection to use the local proxy. I don't think you would get a clearly noticeable speed up, as it uses a generic lossless ZIP compression for every type of content. Most images are already compressed using a lossy algorithm (JPEG encoding), so a lossless algorithm won't compress them any further. At least you can try it for free:

http://toonel.net/