In that case, I'd probably opt for an Intel CoreI5-750 processor. What motherboard I'd recommend would depend on your case (does it accept ATX form-factor boards or just Micro-ATX?), what other internal cards you need installed (is the ATI TV Tuner internal?), and what interface they require (if the tuner card is internal, what slot can it be installed in, PCI or PCI-Ex1/x16?).
A CoreI5 system will probably put you around $450 (including 2x2GB of DDR1333 RAM and an AMD HD5670 graphics card). You could probably save between $50 and $100 by going with an AMD Quad-Core system, but again, I'd have trouble recommending a motherboard.