Well, I said this to Colin a little while ago.. the audio quality on Skype is I think better than most regular SIP providers, so I don't think it will improve the sound quality, but the overall quality of use should be better as it's more telephone like and doesn't require a computer to be on/signed in/etc.
I don't know how bandwidth hungry Skype is, but I know g.729 only requires about 8 - 10 kbit/sec. You would think anyone would have that to spare but it's still entirely possible that the broadband connection is just useless and ultra high latency or something or as gene suggested totally saturated.
I suppose it would be worth trying a SIP service out using a software SIP client like X-Lite before going and buyer a hardware SIP device (i.e. the ATA or SIP phone).