The best card by far is the XFX 4650....and its not that great....
The best motherboard is the ASUS....
No you won't fry the motherboard with a weak PSU...however you might have a no boot, or a boot to a black screen....sudden shutdowns....ect.....A better solution since you plan on getting a card would be to get a decent lower end PSU
Newegg.com - Rosewill Green Series RG530-S12 530W Continuous @40°C, 80 PLUS Certified, Single 12V Rail, Active PFC "Compatible with Core i7,i5" Power Supply Not the best psu in the world, but its doing most of the heavy lifting in my system for now....no problems, and the computer will not shutdown if the power "blips"....and it will leave you with a good backup PSU.....
However I would question spending ANY money on the system you would end up with....AGP is dead....P4 systems WILL suddenly die on you...and I speak from experience on this.....A better solution would be to put together the best that you can, with what you have, and save up for a new rig....A cheap Walmart E machine would smoke anything you could make from those systems, and have a PCIe slot...if not the PSU to back up said slot.....
If you plan on using Windows 7 don't use a OEM copy...or you will be calling up Microsoft trying to get activated on a new system.....
Ready boost would help, but those old systems most likely were USB 1.1.....