Re: Buying a new graphics card
I would go ahead and get a new graphics card (anything PCI Express, which is 99% of graphics cards these days, will work). Install it, if the graphics card has PCI Express power connectors (the 6-pin and 8-pin ones) make sure your power supply has those connectors. If it doesn't, make sure to buy a graphics card that is bundled with adapters (they usually have adapters for 4-pin HDD power connectors). Try it out, push the card to its limits by playing some intense games or running Folding@Home. If all works fine, you do not need a new PSU, if it locks up frequently or BSOD's, you should buy a better power supply to power the card.
A long time ago, I bought a then-new Radeon X1600Pro for my HP 734n (Athlon XP, 200W PSU). Everything I read said the card needed 300W or more so I bought a 500W PSU as well. Turns out everything ran fine on the stock 200W PSU and I wasted money on the new unit (though I am now using it in my server and it does a good job). Also now I am running an i7 and 2 5870's on a 650W PSU without issues.