The problem with bttv cards is typically pci latency. The card needs a high pci latency value (the amount of time it is allowed to transfer data over the bus, this allows to transfer a lot of data in one cycle). You'll probably soon find out that other cards (especially audio) like the value to be low (they like to transfer a small amount of data, very often). The trick is to find the right value in the middle that keeps all the hardware happy. PCI latency can be adjusted on some motherboards in the bios. For the rest there is powertweak.
Tip: pci latency is an 8-bit value (0-255), but the lower 3 bits are ignored. You can save a lot of time when experimenting by adjusting the latency in steps of 8.