A stellar pair of headphones is a real good idea. Right now I'm using a pair of vic firth headphones. You can find them at guitar center or any such place that sells drums, since they are made for good isolation in high db. You can't hear anything outside with them on, and in turn, they aren't letting any sound out. That alone should kill most echo issues.
I also highly recommend the use of a condenser mic with a pop filter; for nothing more than clarity. Skype can sound real grainy, and my calls lost about 80% of "what did you say" and "your cell phone must be cutting out," when I started using some of my recording equipment, as opposed to the mic that was thrown in the box with my pc.
As far as voice modulation goes, there's plenty of cheap guitar pedals and stuff that can help with the right adapters and cords. I still swear by this ineko thing, it seems like it was built for nothing more than phone calls, since I can't really imagine any other practical application in a music environment. I picked up mine after haggling the salesman with doctored html files from other places that sold them. I'm fairly certain that the larger guitar chains, like samash and guitar center have a bottom price of somewhere around 60 or 70 dollars. It's been years since I picked it up, so I don't remember, but regardless the effects are pretty clean and very functional.
