I currently have a 400W power supply that came with my vaio PC. What is the most powerful, sub-$100 graphics card (preferably nvidia) I can get that my psu can power?

Here are the relevant specs on the unit:

+12V1 /10A
+12V2 /17A

Total combined power on +12V1, +12V2, and -12V can't exceed +300W.

Something along the lines of the 8800GS 384MB $90-$100 after rebate seems to be what you're looking for.

You didnt mention if the PSU has an available PCI-e 6 pin power adapter. You may need to use a molex to PCI-e power adapter/power cable.
Make sure you have two free 4 pin molex connectors.

I do have a 6 pin lead, but I was under the impression that my PSU couldn't supply enough power.

What else are you using your PSU to power in your system?
Do you have the make & model number of the PSU? How old is it?
And why were you under the impression that your PSU couldnt suppy enough power?

Another forum member had said that the PSU in my two year old sony almost definitely wouldn't be powerful enough, even though it is 400W.

300watts is 25amps. Thats not too shabby. And at only 2 years old its probably still has most of its rated power.
Unless you have lots of case fans, hard drives and optical drives it should be enough.

In this example system with an 8800GS used 221watts under full load with an overclocked Q6600 CPU.
http://www.tweaktown.com/reviews/1370/18/page_18_power_consumption_tests/index.html

If that type of info isnt enough for you - try hanging out in the Sony support forums and see what others with the same model PC are doing for upgrades.

Okay thanks for the info!!!!!