See, there's your problem. My old server had a p4, and going to a full 1g from a 128 mb card, had no effect. The reason? The processor and memory can only handle so much from the card without slowing down. In other words, the graphics card can feed information to the processor faster than it can handle it. It sounds like that's the case with your setup. You can put whatever card you like in it, and it'll only go so fast. The only thing you can do is upgrade the motherboard and processor/ram.
I don't know what version I have on my cab, but when I built it, I OVER built it! I have a 1g ddr3 card but, I have an I-5 with 16g of ddr3 ram, so I can put any card I want in it. (I can support 32g ram, but it's overkill).
