Check out the Kawasaki Vulcan 500. Decent power and comfortable. Also about the same size as your GS400. But if you can keep the GS400 going I would definitely do that for as long as possible. It is a better bike than anything made today.
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped