Hardware generally doesn't have much to do with your internet speed, its all about what you have running, and how quickly it can process the incomming info.
Also, you have to remember that net speed is measured in kilo-bits (thousand of 1's) per second, where as your network card which attaches to your modem/router is in mega-bytes (millions of 1's in groups of 8.).
However, if your running say a p3, with 256mb of ram, Xp and say Norton internet security your processing speed is going to be well, rather crap.
At the end of the day, most bottle necks come at the Modem/Router. However there are a few things you can do to help increase the speed.
1. Turn off Automatic Updates, this will stop you XP/Vista polling the M$ servers looking for updates etc (make sure to turn your office ones off to), or set to manual
2. Turn off your program auto updates (Java, Anti-virus, Acrobat, etc) again same polling thing, or set to manual.
3. Check the settings of your web-shield or what ever your anti-virus calls it, as it will check all data coming in before it is released to your machine, and add CC to the list of trusted sites/zones.
4. Make sure you don't have any P2P software running,
5. Clear your cache, and check your cache setting. (I'd get a program called CCleaner for this as it does a better job than anything that M$ ships)
6. Blow out your heat sinks (seriously, go down to your hardware store, by some caned air, whip your cover off your computer and blow the shit out, its amazing how much performance can be recovered by having a machine that's not over-heating)
A good way to check on how much stuff is using your internet connection is to open your command prompt (XP start -> run -> cmd, VISTA type cmd into the search box) once its open type in netstat -an if there are more than say 25-30 lines then you have some internet issues.