This may not be want you want to hear but I think you should wait to see what sort of performance you get with your new broadband connection before you start thinking about bandwidth limiting. After all, you're about to increase your downstream bandwidth by a factor of 10. 500kbps isn't a lot but it should be enough for you and your wife to co-exist happily while surfing. Whereas 50kbps just isn't really enough when the bandwidth is shared.
Not to mention what your wife would say when she saw your PC giving good response and hers behaving like a dog
Steve.