its not only hardware but also software(PVR) & drivers that really determine picture quality so try to stay away from generic tuners from tech com,intex,frontech etc........as they have crappy drivers & PVR software
i have used many tuners from generic SAA7130HL to compro videomate tv gold plus 2 9bit SAA7134based to conexant 10bit ADC based leadtek winfast tv 2000 xp global
IMHO consider buying 10bit ADC based leadtek winfast tv 2000 xp global for 2500 bucks .....its has excellent driver support plus its PVR is updated very frequently .......plus its remote controller natively fully supports POWERDVD 5,6,7