I help in buying a good router to play SCIV on
DSL and cable used to be rather neck to neck a couple years ago. However, cable companies increased the benefit of a cable connection - so typically DSL is much slower than cable; altho, in some areas, may have the same or better ping response times than cable, which was the gamer's defending arguement for quite some time (not to get into infrastructures).
Considering a ping of 150~ is considered 'typical online' and that SC4 would be lucky to push 5KB/s, then considering Xbox Live overhead; I would still find it odd that you lag horribly [with people in your region] even with a DSL connection - unless it's some like $15-19/mo 'lite' connection which usually sucks in all facets conceivable.
What makes you suspect your router? In stating 'buy SCIV better' do you mean 'play'? Are you implying lag with your statement?
I would contact your ISP first. What router do you have?