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.
I think the ATP level of play is rising, the unfortunate parity they had for some years is declining, and they are already eclipsing the women's game after being overshadowed many times since 1997. The WTA seems to now be in a state of flux and the Williams factor is so huge in the quality of the top players. They are injured so often and miss so many tournaments, it's hard to gauge just how good the top players are now. I'm very afraid that parity is already on the increase in the WTA and is hurting fan interest.