In VB6 the only way I know how to call a com object over the internet is to use Microsoft transaction Server MTS and then export the stubs onto another machine then you can call it like a regular Dll... In VS.Net I think the easier way to accomplish this is to use Web Services. As far as calling that VB6 dll in vb7 your going to have to fiqure out all that COM interoperability stuff...
As an alternate solution I was reading somewhere that you can create your dll's in vb6 then compile them with a special compiler from MS so it works easily from VS7 but you'll have to hunt a little otherwise I'm not sure. Sorry if I wasn't much help...