Don't know if it's really possible to compare old from new. The names you listed (Sampras, Graf) aren't really considered part of tennis' (distant) past quite yet, since they both used graphite. I wouldn't say the ATP is getting worse simply because there isn't a 14 time slam winner currently on tour.
Just so there's no confusion, I'd definitely give the edge to Sampras (in his prime) over just about anyone on the planet. As far as the ladies are concerned, I think Steffi would have a much harder time dealing with Serena than Pete would have dealing with Roddick or even Federer.