The Speedtouch 330 is supposed to be limited to 4 Meg or so but you have already shown that yours can achieve speeds in excess of that so I don't think your modem is the problem.
It seems strange that your line has suddenly deteriorated in this way. In order to try and figure out what has happened can you post your line stats from the modem -
Go to click here and download "Dr SpeedTouch"
Install the program and run "Diagnostics".
Right Click the icon in your system tray >>
"Write log to disk"
Now open the log file in IE (its in .xml format so you may have to use IE to open it).
You should now be able to see your line stats amongst a lot of other information.
The stats you are looking for are:-
Please post back with the stats.
Re: routers. A combined modem/router is powered separately from the PC, usually by a power adapter that connects to the mains. Some have USB connections but most are ethernet as this is the preferred method and is made for networking unlike USB. Replacing your USB modem for a router wouldn't stress your machine at all and unlike USB ethernet does not require drivers to be loaded. You simply connect the cable from the router to the PC and the router to the phone line, input your ISP login details and that's pretty much it.
If you don't already have ethernet built in or a PCI network adapter fitted then you would have to source this first. Wireless is another option but you would have to have a wireless adapter to be able to utilise that.