The easiest and cheapest is to drill some holes in the backplate of each pipe. Start with one each and increase the number until you're happy. This will mess with your mixture balance a touch and will worsen as you put more holes in, so you might want to re-jet.
The best, cheapest method is to cut open the silencers and rip out all the innards.
Ideally, you'll want to re-jet after this so your performance doesn't suffer, but that will cost a few quid extra.
Unless you're extremely skilled with tools, it will likely look shit, though.
However, if you want to just get different pipes, keep the originals in pristine condition and sell them.
New Pipes:
I have a set of the Gemini ones from the Italian seller on eBay, for something like £250. A simple search will find them (cannot send link as I'm at work).
Quality varies a touch, but mine fit quite easily and replaced the whole pipe assembly.
These have removable baffles (undo one bolt on each and out they come), so you can have the nice growly sound of a medium-sized dog... or take them out and be DEAFENINGLY FECKING LOUD as you amble on by!
The 650 will never sound especially deep and menacing, as it is only a 650cc engine... but it can still make some impressive sounds.
Other pipes are available, but are more expensive - you can pay up to £600 for some of them and they are not all complete systems... some are just slip-on silencers, so you'd need most of the original pipework intact.