Popular Mechanics (ok, not the most credible source, but Kim Komando did the article and she's got her own tech radio show) did a comparison of the internet boosters (The modem booster type thing) and found that none really make a difference. There's some tweaks you can do yourself, but the info is all freely availble on the internet.
As far as boosting your PC speed, there's lots of articles on that in magazines and online. The easiest way to speed up your pc is to clear out the stuff in your system tray (stuff like music match, the aol quick start, the Sound Blaster control panel, etc) all hog resources, and are usually not needed. Of course, check the software documentation to be sure, but I've found that removing them leads to a nice increase in system speed.