If you can physically fit the graphics card into your motherboard then you do not have to worry about the voltages, check the slots on the graphics card and the motherboard to see if they will fit. I think that they probably will, another problem is, is your power supply good enough.
I had checked it out but; because I have limited hardware knowledge, I wasn't sure.
You say that the card should work at 8x speed?
I have P4 2.66Ghz processor, 1GB RAM and 380W power supply. Do you think it is enough?
You may need a larger PSU, the ATI HD 3000 and HD 4000 AGP cards will have to have enough power to function. I don't know what hardware you have installed. There is a helpful PSU calculator here, where you can enter the HW you have, adding the power requirements for the GPU you want. Compare the calculator's estimate with the 380 watts you have. If you need a new PSU, don't skimp, get one that is at least certified 80+ efficient - those have several features that provide "clean," stable power.
If you can physically fit the card then it should work at 8X speeds if the motherboard supports this speed. 380 watts is to close to call as to if it will work or not it will depend on lots of factors. I expect that the computer will work fine with this power supply but when you try 3D graphics you may experience crashes due to the power supply, try it and see.
It will run at 8x, if your PSU is sufficient. I only brought up the 4X/8X because that is a standard showing that those cards do not require 3.3v at the AGP slot. That is also likely the reason those cards will need a 4-pin or 6-pin power cord from the PSU - to provide the power needed for the card to run.
Don't worry about the power supply; it's definitely big enough. An HD 4650 or 4670 will definitely work on a 380W PSU -- I personally have one running on a 300W PSU and have heard many similar experiences that confirm that.
Both of those cards are going to be far superior to the 3450, which I would not recommend buying at all. DDR2 and a 64-bit interface is not going to give you good performance. The DDR3 and 128-bit interface on the 4670 is basically going to be the max upgrade possible for your system.