N Router with G adapter how much you would lose?

Unless you have fiber optic internet you shouldn't really notice a difference in normal use.
Internal network is more affected though.
There you may actually notice a difference.
As it stands most average web interaction is limited by hubs and site servers, generally they won't pass 5 MBps regardless of whether your connection can handle more or not.
With the internal network, you are dealing with 54mbps vs 300mbps.
Now I never finished college but I'm pretty sure that 300 is bigger than 54.
Though honestly most wireless n I've used is limited to about half of that either because of signal strength or some internal issue of some sort.

anyhow, I guess what I'm saying is what is lost is situational. It could be nothing it could be a noticeable difference, it mostly depends on the connection to the net/network and what you are doing at the time.

You should have a 150mb connection (maybe more maybe less) when using full wireless N yes. It loves to adjust speed for some reason... a lot more than G did anyhow.

The reason I mentioned fiber optic is it's about the only internet that would be fast enough to push past the 54mbps wireless g limit to begin with.

I've honestly never noticed any real difference in my internal network (though I only stream lower grade video I usually just run HD stuff locally from the machine in the main room, and since my internet rarely sustains a speed above 2 MB/s It doesn't really come up there. I would say on average the n is about 100kbs faster than the g in that scenario. ...that could just be signal strength though.

In my experience where wireless really fails is with downloads where a lot of connections are in play, like with a torrent or p2p download scenario. I've had results from both N and G that include, loss of connection, loss of connection data resulting in no connection, significant choking when many connections are active at once regardless of signal strength, high number of failed hashes, and other such annoyances when using wireless for p2p. Though it does not seem to suffer the same problems in a usenet scenario or normal download/stream where there is only one to four or so sources.

Is there really a "normal"? It depends a lot on the server that is sending/receiving your data. I noticed that uploads to some sites are nearly at the rated speed whilst other sites are super slow. And downloads are a similar problem. I remember Vista SP1 which took an awfully long time to download. Then MS had installed some fast servers (after a lot of people complained) and I downloaded the whole SP1 in 12 minutes on a DSL 6000 line. If I remember right it was about 460MBs.