You should be okay installing the HD 4670. However, it seems that your system is a bit old and the general rule with PSU is that they tend to become less efficient and provide less power as they get older.
The HD 4670 is probably the best video card for AGP. nVidia partners have stopped making AGP a couple of years ago. Whether or not an ATI partner will create an AGP version of a value HD 5xxx card remains to be seen.