That’s happening to me too, I have a Asus A8N-SLi Mobo and a Gainward 7800GT 512MB - make sure all power connectors are rightly connected to the motherboard. This is what I think my problem is as Im 4-pins short
I think it will be the power supply, you shouldent even be running the card in there with a 250w power supply, i think nvidia will reccomend 300w min for the graphics card, also what is thye make and model of that power supply?, i rekon the 12v rails on that 250w power supply will have low amps.
i think you should defenitly buy a new power supply, especially if your going to think about upgrading that computer even further!
you shouldent need to be setting any jumpers on your motherboard for it to work :s
the only jumpers you should ever have to set, are on cd rom’s and hard drives, configuring them for master or slave, there is some on the motherboard though, for sound and other things like that, but not for graphics, unless your motherboard is super old!, which it is not becouse its PCI-E