Your computer seems able to handle an upgrade to a 256mb video card. I am slightly confused on your mother board specs.
Also, dump the 430 watt power supply and get at least a 550 watt or better by a named brand. Like Antec, Thermalright, Cooler Master, and such.
I use Corsair 3200 XMS dual 1GB memory and have no problem playing any game I want at high settings. Now my video card is a ASUS AX 800PRO/TVD 256mb and has some extras.
When playing Doom 3, FryCry (a great test for your video card), Half life 2, and FEAR I have high video levels with no garbage. Quake 4? Have not played that yet.
Please remember if you are changing/ upgrading your computer cooling will also change. If you get a 256 mb video card, you should also get a Vantec Fan card to cool it.
Cooling should be your first priority when gaming. Do the work now and play later. Or we can help you repair your computer when it dies.
Let us know,
Your Hardware Guide.