well generally depending on the make of the psu, you should be looking for at the least 22amp on the +12v rail. However it is highly recommended that you save yourself some trouble and go for something with 28-30 amps.
Make sure its a good make as some crap psu's are 500-600 watts overall but only have 12amps on the +12 rail. You should be looking for about 500 watts with 28-30 amps on the psu.
The thing is my system is about 1.5 years old and obviously running a agp port it came with a 9800 i upgraded it to a x800xl after a couple of months.
That died recently and i noticed the 1950 but alas i have chacked and have a 400 watt supply so dont think i can run it any ideas to get a bit more life out of the system before i bite the bullet and upgrade.