I am ready to upgrade the video card in my PC, but just discovered the PSU is just 250w !!! I have an HP a250n with a P4/2.6 and 1.5gig RAM installed. Will an nvidia 6600 256MB work for me with this power supply? Is this a good choice for a card? I really want a 6800 GT OC (no way, right?) Can I change the PSU? I was told by a store sales guy that the mother board is designed for this PSU and if I try to increase the power.... I could blow out the board. Is he right?