How bout this. Upgrade the power supply to 450 or so, I suggest Allied. Radeon 9800 Pro 256 (It's what I have). And 512 more RAM. If your first RAM is PC3200 then make sure you get PC 3200. I recommend Crucial. It's expensive but try
www.newegg.com Then you have basically what I have (My CPU is a 2400+), and it can run Doom 3 maxed out, no lag.
I am, as you should if you want to, going to upgrade the processor to 64-bit. I suggest an AMD 64bit 3400+, with the new Asus motherboard. If you're going to do it.