you need to leave the NV card's latency alone. That is really a reflection of the AGP bus as oposed to any real competition for PCI bus time. Aslo, will probably find that it will get changed back even if you try to set it lower.
As for your NIC, is surely does not need a latency of 128 so I agree you can play with this.
Powerstrip is fine alongside Rivatuner. Just pick Rivatuner to tweak your drivers with, it is better than powerstrip for that IMO, though I prefer to overclock with Powerstrip.