|"The PSU was/is a concern, but I keep finding threads that they are generally under-rated"|
You have to take any info posted in a forum with a grain of salt. There's a LOT of misinformation out there & a lot of idiots that continually spew it out without ever verifying if it's correct or not. If it's not backed up with some sort of proof, be suspicious.
How so? As in a 500W unit being marketed as a 350W unit? Why would a manufacturer do that? The PSU is arguably THE most important piece of hardware in a PC...every single component depends on it. If your PSU is underpowered, you'll experience random shutdowns at least, complete system failure with fried components at worst.
"Any suggestions for an nVidia chipset card?"
No, not without knowing your PSU specs. I'd need to know the brand name, the wattage, the amperage specs for the +3.3v, +5v & +12v rails. The HD4670 will work with your current PSU...anything else is a gamble.
Do some reading:
Power Supply Myths Exposed!
A short history of PC power supply voltage rails
Everything You Need to Know About Power Supplies
Best PCIe Card: $50 To $110
Mainstream Graphics Card Roundup