I recently upgraded the graphics card from an Nvidia 6200SE to a ATI (ASUS) HD 4350 512mb. After the switch the pc has painfully slowed down. At first I thought it was a drivers clash so I reinstalled Windows but its still just as slow. Both cards don't need to be plugged in to the power supply which I believe is 300w.
Build your system into this to give you a better idea of how much power you require, http://www.thermaltake.outervision.com/
But given that you probably have a lower quality psu anyway, I would suggest a new, more powerful model,
Have you upgraded the ram at all or are you still on the original 1Gb?
having the extra ram in there is going to help speed things up,
and the power required for ram is negligible although if the sticks are mismatched, they will both be running at the slower sticks speed, I would invest in a new psu, even though your current one says 300w, it most likely isn't putting out anything like it, more like 250 if your lucky and age is a factor too,
if you post full specs, we can look to seeing what else may help you
Try running the rig just with the 1Gb stick in,
and check scan.co.uk, ebuyer.co.uk for psu's, I used a cheaper brand for my Gf's WoW pc and its fine, little low on the 12v rail but her 5770 is powered fine, http://www.scan.co.uk/products/600w-storm-storm-600w-bl...
its a case of what you want to spend on a psu though